Quoted from rwmech5:Naptha, micro fiber cloths, coil stops and coil sleeves. Pinball Life has some small hardware kits as well.
I second this and would add nuts drivers, telescoping magnet and a leaf switch adjuster. These are things that can pretty much be used on every game. I would say some new balls as well, but depending on the games I would avoid certain ones. Oh, and a bottle of novus 2. I gave my brother a bag of stuff like this last year.