If you are a Harry Potter fan, you'll love the tour. If not, you'll think it is a total rip. It will be crowded as Hell. Bring snacks unless you enjoy paying top dollar for food. Book early and if you get a late tour, show up a couple hours early, they will probably let you in any way.

I didn't do the Eye or go on the Eiffel Tower. The lines are nuts and a poor use of time. Climb to the top of St. Paul's Cathedral (check out the ancient graffiti along the way) for a great view of London. If you bring some nice clothes (dress code), London Sky Garden is free before 10 and that is an even better view, plus they have elevators. Notre-Dame and Arc de Triomphe provide great views of Paris.

London - Do a bus tour early to get a feel for where everything is. I really enjoyed the museums and cathedrals. Churchhill's War Rooms was a bust for us. Shakespeare's Globe Theatre tour was very cool, but mostly because of the Cockney tour guide. The parks are amazing. We really liked Kew Gardens. Eat a hearty English breakfast at a local place for ~5 pounds. You are going to do a lot of walking. Wear good shoes and fresh socks.

Paris - If you are taking the Eurostar from London, be prepared, the area around Gare du Norde is sketchy AF. We survived though. If you don't speak French, try to learn the polite words. Most of them speak English and if you just try, they will help you out. Amazing museums. Check out the Catacombs (early in the day) for something different. The French are not big on breakfast, so we'd grab some bread the day prior. Fancy bread is dirt cheap there. Beer is cheaper than Coke. Again, lot's of walking.

We did a week in both London and Paris last year. London rents are expensive. Paris is actually somewhat cheap. Get copies of Rick Steves' guides to both cities. They really come in handy. We used Airbnb.

Who's your cell provider? If T-Mobile, your gonna love it. Just confirm before you leave, international service isn't blocked on any of your lines. I had one line that was blocked for some reason and it was a pain in the ass.

Have fun!

Also, I've known a couple people who have done Disney Paris and would not recommend it. An amusement park is an amusement park and we have so many in the states. Take advantage of the time to do the stuff you can't do in the states.

Congrats Rai.. we're headed to London too this summer.. I agree a bus tour is touristy but it's helpful getting a preview of the area and most of the guides are funny too!.. It helped when we went to Edingburg, Scotland.
Taking wife to theater in London.. and probably plan a day trip by train out to Bath or Cotswolds..
rest of the time at Museums/parks/out to eat and just walking around...
I'd give you advice on Germany.. but we were there way south of Berlin in Munich..

There's so much awesome stuff to do. Why waste time on a theme park which we in the good old US of A do better than anyone? If it means a lot to you, certainly do it, but when I break down my cost per day for that trip, Disney is the last thing I want to do.

London

Tower of London - Do one of the tours led by the Beefeaters there, well worth it rather than trying to do self guided, you learn a lot more.

Westminster Abbey - Again, take one of the tours by the volunteer docents there. I think there may be a small fee and you may have to book in advance, but it’s well worth it.

Covent Garden - Go to watch the street performers. The pub there has a balcony, if you’re lucky or early enough to get a seat on it, it’s a great place to have a pint and watch the performers.

Leicester Square - If you’re into theater, there’s a half price tickets booth for unsold tickets for the same day. You likely won’t see tix for major broadway type productions that are well sold in advance but you can get still get some good stuff. If the weathers nice I highly recommend the Open Air theater in Regents Park. You can buy bottles of wine and food there to drink at your seat outdoors while you watch the performances.

Don’t miss the Sacre Coeur in Paris. It is beautiful and the view of the city is also great from there.

In Berlin you can find a lot of museums about DDR times, which I found interesting. I guess it would be even more interesting for someone who did not experience anything similar.

Keep the ideas for London coming! I'll be going there for a week in the summer with my wife and two kids (10 & 13).

Not to derail the thread, but we may head into Brussels for a couple of days too.

All kids visiting London must go to the London Dungeon for sure.
London Eye is a must and quick/easy

Kids probably too young for the Globe Theatre
Madame Tussauds can be skipped

In the summer? I went in early June, supposedly before the busy season, and it was still nuts. Plus if I can get 90% of the experience for free vs. ~$130, I'm going with free.

I think it was in May...wasn;t much of a wait.

Ahhhhh right, didn't have kids then forgot to add that price in

He will have plenty of opportunities to spend money. Any break you can get, take it.

The HMS Belfast in London is also interesting, especially if you have boys

Still trying to wrap my head around how somebody can go to Disney "hundreds of times"

We like to get souvenir t-shirts when we visit places. Sizing in the UK is generally pretty comparable to the US. France? Forget it. I upsized and it was still too small. If you do the Globe, they have a great selection of t-shirts which were apparently designed in France.

Unusual things...may or may not be your taste.

Leeds Castle, or another....Hedge Maze preferred for the kids.
Victoria and Albert Museum. Stuff there no where else.
Catacombs, or Roman Ruins in London.
Underground tours of Paris
Novelty Automation in London, fun.
Covent Gardens, shopping, London
Kew Gardens. Superb.
Eat daily specials in Local Pubs.

Give me a shout when you are in town, sometime.

As a Londoner, yes, all the usual attractions but be sure to check out the university towns of Oxford and Cambridge, also. Not far from London.

As someone else mentioned, the Cotswolds are well worth a visit, also.

Roger

No need to justify it. If you enjoy it, do it. I'm an hour away from WDW and Universal. Disney has gotten too damn expensive for me, but we've been doing Universal annual passes for years. I yearn for the good old days of "buy a year, get a year free", and they nearly lost me this year, but I really like the new water park and they are offering 15 months for the price of 12. If we go to at least a couple of the "free" concerts, it's hard not to justify it.

Kew Gardens feels like you are walking around in an early Disney movie. Also, check out CityPASS. We did it for London, but not Paris. Some cities it's a better deal than others. London Tube, get a card. Paris Metro, it's cheaper to just buy a book of tickets (don't wrinkle them).

https://www.citypass.com/

The standard tourist things are recommended and I'm not repeating below. Additionally I'd suggest

In London
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Borough Market - a giant outdoor food market. Go hungry and make a meal of it. If you're a foodie you'll love it. If you're not a foodie you'll still enjoy it.

If you like antiques - Portabello Road

Be sure to eat Indian food if you like it... and try it if you haven't.

In Paris
=====
First... get a multi-day metro pass. You'll need a picture and can plan ahead for this. It is well worth it.
Second... get a Paris Museum pass. It's worth it just to get in the "fast" line to enter the louvre... but also for the other things it works for.
(See Rick Steve's for more suggestions... they were all very good)

Add the Musee d'Orsay to your visit. It is a great small art museum. Actually liked it better than the Louvre

Add Sainte-Chapelle (church) - it's a gorgeous church near notre dame with insane stained glass

Add the holocaust museum - also near notre dame. Short but worth a visit.

Lastly, the one thing my wife and I regretted was not planning out some great French meals (restaurant reservations, etc.) in advance... we always were out and about and ended up eating where ever we were. Food was great but we didn't get a fine french meal.

The Latin Quarter (Paris) was good for reasonably priced food and shopping.