What to get a 6-year old city boy?

My only nephew is turning six next week. He already has waaaaaay too many toys. I want to give him something memorable. I wish I can just give him baseball bat or something, but he lives in the city in a really small apartment with no backyard.



My first thought was to get tickets for his dad and him to see Cirque du Soleil, but this will put a big dent on my paycheck (as much as I want to spoil my nephew).



My second thought was tickets for his dad and him to go to New England Aquarium.



Does anyone have other fun ideas of what to get that will not kill my wallet? My budget is $100 max. Thanks for your help!What to get a 6-year old city boy?
Something that will spark his interests. The best thing I ever got was a bumper box of all the Roald Dahl books because he's my faviourite author, and I loved to read, and want to be an author when I'm old enough. That kind of thing :)What to get a 6-year old city boy?
Maybe a year-roud pass to Central Park or the Bronx Zoo or something if he lives in NYC.

I was going to suggest a bike for him to ride around his neighborhood, but that might be a little old for him.