This is a very popular area and gets a lot of visitors, but you could be lucky, walk up and get a tour, or you could have to wait hours, or not end up with one at all. The same applies to lodgings, most people who "wing it" without reservations usually find something, but the lodgings in [and close by] the National parks can, and do book out in advance. Personally I would book, it's a short road trip and quite straight forward in scheduling, but that's me.

Good luck.