We've always booked a few ADRs (fewer and fewer as the years go by), but I can understand why people would rather be spontaneous!
I've always thought the onsite CS restaurants were good options (some of them I'd even call "great" options), the app makes it easy to see if any TS reservations are available on the spur of the moment, and if you have a Landry's Select Club card, you can always walk up to Yak & Yeti, Rainforest Cafe and T-REX anytime you want and get priority seating even without a reservation. While many people will breathlessly tell you that this restaurant or that one is a "must do," it's really not. I never stepped foot into any WDW restaurant (CS or TS) until I was a full-grown adult: all of our family trips when I was a kid had a shoestring budget, and we brought and ate our own food at the campground, yet I never felt our vacations suffered one iota by not having restaurant reservations! In fact, my own family's vacation planning, I've never regretted not making a reservation somewhere, but I've regretted having my schedule dictated by dining reservations on several occasions.
*Note: I see you mentioned "December." If you're planning a visit near Christmas, there will be lots of competition for dining venues of every kind. If you don't care to make TS reservations, be prepared to eat CS instead, and make use of mobile ordering (where available - your My Disney Experience app will have all the information and a link to order) to minimize your waits!