How lucky for you that he likes his potty so well <wry smile>.
I think your sponge bath on the towel is a good compromise. You're still changing his diapers, anyway, so that area is clean. Doesn't hurt if he gets a little dirty in some other nook or a cranny here or there while he warms back up to the tub in his own time. But if you want him back in the tub sooner than later, you could get him a neat new bathtub toy. Instead of bribing him with it, just stick it in there and casually get him in there so he can "accidentally" just happen to notice it. When he wants it, it's bath time. Better yet, make it a toy that suctions to the wall of the tub. You might just find he's climbing in, himself, and you have to get him to wait while you undress him :)
I really don't recommend any kind of bribe, because that gets it into his mind to ask, next time you want him to do something, what's in it for him.
L.
PS BE CAREFUL. Soft plastic toys are toxic and have chemicals in them that can feminize boys. Especially hot water will cause those chemicals to leach out of the plastic. Look for natural, organic, phthalate-free toys. You could go with something natural rubber. Here are some URLs I just sent my dad for my daughter's Christmas present suggestions:
http://www.naturallytrendy.com/item_579/Cool-It-Soother-t...
http://www.oompa.com/baby-toys/category.cgi?item=HA1121
http://www.oompa.com/baby-toys/item/HA1296/Haba-Trix.html
http://www.naturallytrendy.com/item_660/Chan-Pie-Gnon-Nat...