Fishing, hiking, and campfire socializing.
Come rain, snow, or sunshine, I'm a tabletop gamer and am most at home sitting around a table with some friends - making up stuff and rolling dice. Most of my other hobbies are peripheral to gaming - reading gaming books and ideas for stories that other people came up with, collecting and painting miniatures, brainstorming modifications to rules, etc.
But I can't do that all the time, so I do other stuff, too. This summer I'm doing a lot of traveling - meeting up with long-distance friends and checking out the sites in other parts of the country. I've been spending a good amount of time on planes and in terminals, so I'm reading more. I also do some computer gaming - mostly RPGs and puzzle games.
I love camping, hiking, and all that outdoorsy stuff, but I don't seem to get around to it as often as I used to. My cousin and I installed a fire pit in my backyard a few weeks ago, so that'll do in a pinch
Swimming at night or when there's a canopy over the pool (I burn like a vampire even with sunscreen so my doctor said I can only swim or be outside in indirect sunlight) and camping.
We used to go camping every summer, until my accident. It was so wonderful, but we haven't gone in prolly 8 years now. I miss it, but it's so much work.
My favorite thing about summer, and believe me there aren't very many, is watching the hummingbirds. They are so beautiful. They are like watching flying jewelry.