My 3.5 yr old son is all hot wheels, all the time. He has tons of cars but I'm adding lots of tracks, ramps etc to our amazon wishlist.
He seems to be starting a fascination with superheros. Spiderman seems to be a favorite. So perhaps it's time for action figures?
He likes Playdoh and all the various rollers, cutters, shapers etc.
I read some of the other answers as I personally am mighty sick of hot wheels. We have a lot of the other things suggested so I'll just add my observations. Of course all kids are different...
Mine is not much of a builder. We have 3 types of blocks - large cardboard blocks, foam blocks, and wooden blocks. He only uses them to make car ramps, tracks etc. The large cardboard ones are his least favorite.
Some specific books my son really likes (that we like too) are:
Library Lion, by Michelle Knudson
Miles to Go, by Jamie Harper
Ladybug Girl and Bumblebee Boy, by Jackie Davis
Duck on a Bike, by David Shannon
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Now I've thought about this a bit more. My son like puzzle, but really prefers floor puzzles to table puzzles. And while I start with the border, he starts with the face. So floor puzzle with faces. We have a 36 pc Curious George puzzle, a 24 piece Thomas the Train puzzle, and a alphabet train puzzle that he likes.