personally, i'd probably never ride with sandals on my feet, but why don't you just put on whatever sandals you prefer? flip-flops seem like kind of a stupid idea because they will either fly off of your feet or you will hurt your feet unless you have a brake on your bike. and if you have a brake on your bike, why not just put some flat pedals on and then you can ride whatever type of footwear you want anytime you want.