In my opinion, you could check out what kind of courses they typically offer in their curriculum at MIT/etc and check out their corresponding courses on OCW. Since you're doing this out of sheer fun, don't be bothered getting too caught up with the structure of it. If you see anything you like, just read up on it.
Usually the ones you've mentioned are a good stepping stone towards more advanced ones though you mentioned you don't want advanced ones. Here are some you can look into:
1. user interface designs (helpful with any startup I think) 2. software engineering (teaches you a little bit of structure in a development process) 3. courses about testing, testing methods 4. operating systems (not crucial but it's fun to read, kinda helps with ideas about distributed computing)
Good luck!