if you are going during very busy times (such as holiday weeks) then I suggest you get at least some reservations or you will probably be eating all fast-foods. It can be daunting to plan out...but you can guesstimate which parks you might be at (calendars will be out 6 mos in advance so you'll have good idea which parks will be open late on what days) You will find that most Disney restaurants offer a variety of menu items so not knowing what you're hungry for 6 mos in advance is not that big of a deal. For example, even a place like Coral Reef that's basically a sea food place has steaks and entree-type-salads on the menu. So chances are wherever you book will have something that appeals once you're there.
If you're not going at a spectacularly busy time then you can find availability at resort restaurants, usually on a walk-up or same-day-call basis. NOT character meals though...would not count on getting a walk-up at a character meal, even during slow times (not to say it cannot happen, but your chances would not be good.) Even during slow times you probably will not get walk-up availabilty at in-park restaurants, especially very small/very popular places like Le Cellier, Coral Reef, etc. Going at off-times, especially late lunches, you can usually find some park restaurant availability but not the most popular places.
We've been to Disney numerous times over the last 3 years during January (after holidays), also several times during
Free dining in September (Labor Day holiday week) and also in May, both before, during & after Memorial Day weekend. At all three times (yes, even during Free Dining/holiday week) we have seen quite a bit of availability at resort restaurants...varying from a few empty tables up to mostly empty restaurants. And we had good meals at Kona Cafe, Grand Floridian Cafe, Olivia's, Maya Grill, The Wave,and Captain's Grill. If you like exotic, Sanaa at AKL is interesting. Other places to try are Raglan Road and Wolfgang Puck Cafe at Downtown Disney. If you're doing the dining plan, all of these places I've mentioned are 1 TS credit meals.