How can I break up a small boulder without power tools?
You can go at it with a sledgehammer (or a smaller hand sledge) and a star drill, and then drive wedges into the holes (or if you are patient, fill the holes with water and let them freeze in the winter.) There are special wedges designed for use in round holes for splitting rock (feathers and wedges seems to find them). https://en.wikipedia.org/wiki/Plug_and_feather

Remember to use ear and eye protection.

Another approach from our practical forebears if you just need it out of the way would be to dig a hole 2-1/2 feet long, 2 feet wide and 3 feet deep next to it, then roll it in and bury it.

You could also build a bonfire on or around it, but that has some potential to become more exciting than planned for if the stone has trapped water in it.