Really whatever's the most comfortable for you! I wear flip flops a lot when we go, and they're comfy for me, but I know that a lot of people wouldn't be comfortable in flip flops.
Crocs are great for the parks, but I've found that mine rub the side of my foot if I wear them all day, which leaves a blister...you could wear socks to eliminate that, though.
In the winter, if it's too cold for flip flops, I wear running shoes. Just avoid wearing shoes that are new and need to be broken in...I've made that mistake before, ouch!!
Pack a pair of really comfy slippers or soft, fuzzy socks for when you get back to the hotel. Your feet will be so happy!!