I'm asking for halloween costume ideas for a 4 year old boy?

Halloween is the only time of year that i wish i had a girl. lol I find that when doing a search for halloween costumes for boys, you get the basic super heroes, elmo, fireman, pilot, and that's it. then when you look for girls theres all KINDS of things. so my questions is...... What are your suggestions for my 4 year old son for halloween? He's really into sharks, fish, lizards, penguins, kung fu panda... lol Any sites you can recommend for any of these? or i'm pretty crafty, so if you can give me any ideas on how to make any of these? you guys are always awesome. thanks a bunch!!!