We were there first week of June last year (5/31- 6/8). I would say if you go during Summer, the first two weeks of June are the way to go.
Parks were on the moderate side (6-8's), but did not feel "packed" or crazy with people. Typical day for us was RD with no to little wait until 10:30 or so, then use FP+ once crowds picked up. We then ate lunch and went back to our resort to swim/enjoy the pool bar - the Pool Games Staff at WL were great! A couple days we started at the pool and then switched FP+ for the evening.
After freshening up in the room, we had dinner. Then we either went to a park and rode in short lines during parades/fireworks, stayed at the resort and watched movies on the beach and the Water Parade go by, do mini golf, or DTD. Whatever we felt like - so yes, spontaneity is still possible at WDW!
The weather was warm - touched 90 in the afternoon, but it did make pool time that much better!