Do you want a day activity, or are you looking for something to enjoy long term? Long term, something like exercising, learning to cook, learning to code, Linux, woodworking, and others can be pretty fulfilling. However, if you just seem to be bored (or slightly depressed) and want to have a day, maybe try going to the library, or a day trip hiking, or something you’ve had even a passing interest in before, assuming it doesn’t require too much preparation (I’m not going to suggest climbing a mountain if you’ve never done that before) or is out of your budget, just try doing it.