My 4 year old boy is ALL about superheros (particularly Spiderman, Batman and Iron Man), Toy Story 3, dinosaurs, sharks, alligators and snakes. Action figures, dress up costumes -really anything involving any of those is a big hit. Big, chomping teeth always get lots of attention! There are some cool Spiderman cars out there and the Toy Story 3 merchandise is really cool -they have the toys and the action figures, but my son LOVES his Buzz Lightyear jet pack and laser-shooting gloves. He wears them all the time.
Not sure how you're related, so not sure how much you want to spend -but my son also loves his Leapster. It's not cheap though -and the games are not cheap either (around 15-20 bucks). He does LOVE his bicycle and has really enjoyed riding it. It's a Toy Story bike -and he has a Spiderman helmet. He also loves his Razor scooter! Those are great because they're easy to fold up and take anywhere.