A few people have said it before, but also travelling. It's just awesome and it's even better when I travel alone (which I rarely get to do, alas). In general, I just love to explore new places alone even if it's just a park I haven't seen yet in my city. There's a hill nearby where our small airport is and I enjoy going there alone and just sit in the grainfields next to a small brush and read or draw or listen to music for a few hours.
I just found out how relieving it is to actively allow myself to fidget after subconciously repressing my fidgetiness for so many years. Probably also the reason I enjoy going to concerts and driving alone in my car so much. Concerts of pop-punk and post-hardcore bands are one place where it's socially completely accaptable to jump and fidget to the music and sing along and jump into a mosh-pit and I guess I'm able to let go of a lot of internalized pressure because of that. And driving in my car alone is nice because I can sing along to my car radio as long and as loud and as much out of key without annoying anyone.
Looking at the ocean, being on a boat, swimming, diving.
Cooking, trying out new recipes or ingredients, eating home-cooked food, drinking tea/coffee/juice/... while sitting on the balcony, having breakfast alone before my family gets up, having some really nice hot (or cold, in the summer) bevarage when I'm going somewhere and decide to have a small break in a café on the way.