Crocs also sells straps with velcro for about $10 that you can use to replace straps that are too floppy. I did this with my Caymans, as they were a little large, but I couldn't wear the Beaches since they didn't fit my feet right (one size was way too long, the other too short).
You can switch the velcro straps around and get a two-tone look if you want.