I'm sure there are plenty of great sandals that are not Keens, but there are different styles of Keens that are more or less bulky. We had the Seacamp ones last summer and they were awesome, but we tried the Newport ones on and those were much bulkier.
I 2nd the seacamp. Way less bulky. They hold up so well. The first year we had H2 and those are bulky. Seacamp is pretty flexible.
Stride Rite makes a pair of sneakers that are similar to Keens in style, but much less stiff and not as heavy. DD1 had these last summer and they held up well. They didn't get too stinky either, although we did have her wear socks with them if she was wearing them to daycare (super stylish, I know).
I held off on buying my kids Crocs for a long time...I thought they were ugly, lacked support, and were just generally a bad idea. I finally gave in a couple of years ago and they've been wearing them regularly ever since. They're great in the rain and at the pool, don't slip on wet surfaces, stay odor free no matter how stinky the kids feet get, and clean up in a snap with the garden hose. Plus while the more "traditional" style I always associated with the name is easiest to find, there are actually some cuter options available if you look for them or happen to have a Croc store/outlet near you. The only thing I don't like is they don't have half sizes, so I can't get myself a pair!