Things that are worthwhile are good — they're worth the time or money you spend on them.
If you walked out of the theater and said "Wow, I'm glad I saw that movie!", then your film viewing trip was worthwhile. If going to the gym helped you get in better shape, it was worthwhile. If hanging out in the park made you happy, it was worthwhile. If something is a total waste of time and you wish you hadn't done it, then it wasn't worthwhile.
