I'm going to go with a superhero theme because I think (hope!) that might stick with him for a little while before he outgrows it. And my plan is to just find a solid color comforter that we can just continue to use once he's moved on to other ideas.
Here are some of the ideas I've been thinking about; his walls are yellow, and he has a red Pottery Barn anywhere chair, so I feel like just doing a general superhero theme with lots of primary colors will be easy to blend into the room.
I only have a few things so far, including these metal, vintage-looking signs that I picked up while in NYC a few weeks ago (gotta love bargains from street vendors!):
Other items I'm considering:
I did these free printables for Jake's bday party and had them on the front door; I'm thinking they might look cute if I printed them out as 8x10's and framed them in white frames?? (just one of each, obviously, not 12!)
This vinyl decal from VinylKrazy is a must ($10!), I love this phrase and it's so perfect for him (I'd probably order in red or blue):
Mainstays Bed-in-a-Bag ($30, walmart):