A cheap, yet impressive, mod is the Iron Monger flasher kit from pinballrefinery.com. A strong led (different colors available) is put behing the Monger and it flashes at the same time as the regular flashers under the Monger. The only disadvantage with this mod is that it needs some soldering. I have installed a red version of this and I am very satisfied with the result.

Another mod that seems great is under cabinets leds from rockcustompinball.com (the standard version, not the basic one). This also works in accordance with the action of the machine and does not require soldering. I have ordered this mod but not received it yet.

I also plan to install side mirrors and a shaker motor in the future.

I would avoid mods that require remotes, batteries and temporary installations. I am also very reluctant to drilling in the toys and/or board.

Another simple mod is the target decal set from Pinballdecals.com which spells I-R-O-N on the left side and M-A-N on the right side. This is something "forbidden" because the license does not allow splitting up "IRON MAN".

Look out for cheaper decals, though...