D.P.
Some simple but fun ideas are:
Bubbles, Sidewalk Chalk, Playing cards, binoculars, Hotwheels or Matchbox cars, Tonka trucks (the small ones aren't too pricey), kids' DVD's like Veggie Tales, 3-2-1 Penguins. Travel toys for his age group should be easy to find, and aren't that expensive. Play Dough kits are usually a hit and are educational as well as fun.
Also dominoes, anything Legos, board games geared toward that age group (keep the slip, so parents can return if they already have it, and then they can get something else).
And amazingly enough, a gift card from Target, WalMart, or Toys R Us, has always been a winner in my experience.