There are a couple different kinds of puffers that might work. I am in the process of picking out a small puffer for my densely planted 10g aquarium myself. (Moving out the SMB)
That being said the best bet would probably be Carinotetraodon travancoricus, which is the dwarf indian puffer, pea puffer...whatever people want to call it. Its supposed to be active during the day, and can even be bred in captivity. Im sure that pufferpunk will have something to say about this thread tomorrow.