Hi M.,
My son was into bionicles when he was younger and I think they are your best way to go. Like you said in your post, your 2 yr. olds like spongebob, so the bionicles will be something he will be able to play with longer. He will grow out of spongebob first.
Just in case you don't know what bionicles are, they are a canister of parts that are put together to make a lego-like robot. There are different "generations" and teams of them, so there are alot to choose from. The great thing about them, as we learned with my son, is that you can put the parts together with parts from a different bionicle and make something completely new, which comes in handy when you lose some of the pieces, as children always do. For about $30 you should be able to get him two different canisters. Each bionicle has his own colored canister, so get two different colors.
Hope this helps,
L.