I was an only child, too, and I spent a lot of time on my own, too.
I wasn't very coordinated, so although I loved to run around, my knees usually had cuts & bruises on top of cuts & bruises.
Often I had my nose in a book. I was and am a huge reader.
I loved arts & crafts, and I'd spend hours drawing, coloring, making dollhouses out of scrap materials, putting together things from kiddie kits, like beaded jewelry or whatever.
I also loved using my brain, meaning crossword puzzles, jigsaw puzzles, logics, and brainteasers.
Now and then I'd write a story or an essay, but more often I played out my stories with my dolls. Skipper ran away from home endlessly and across the country to find her dad (my parents had separated), and my Little Kiddle paper dolls were stranded on an island numerous times. I was into adventure!
I also watched a ton of TV, and I really don't think it was bad for me at all. In fact, I learned an awful lot that I wouldn't have known otherwise, just from cartoons, sitcoms and police shows, not to mention the educational kiddie shows.
When we lived in the city, we lived near a roller rink, so I spent countless weekend afternoons there. Once we moved to the 'burbs, there were more kids around and I was a bit older and allowed to roam the neighborhood. I spent hours riding my bike around. There was a good park a few blocks away, with playgrounds, woods, a pool, tennis courts, etc. Naturally, I spent a fair amount of time there.
My mother sent me off to day camp every summer, and I went to overnight camp a bit, too. At camp, we swam almost every day, either in the pool or at one of Michigan's Great Lakes or a smaller lake's beach. Camp gave me my love of hiking in the woods.
And then there were board games, an essential part of childhood for me! Monopoly, Aggravation, Clue, and my favorite, Careers.