A very partial list. All suggestions are welcome.
  1. Go to the movies
    Sometimes you just need to be able to digest something without running it by everyone in earshot.
  2. Write
    This is a tricky one, because groups can also be excellent, but the peace of writing alone is unrivaled.
  3. Go to a fancy sit down restaurant
    This one is sometimes uncomfortable but is such a good challenge and you just get to sit in the glory of all the culinary perfection you could ever want.
  4. Watch the saddest movies and tv shows you can find
    The catharsis of streams of tears in a room that is quiet except the sound of your crying and the dialogue of your favorite characters? NECESSITY OF LIFE.
  5. Walk/run
    Walking through a neighborhood or on a beach or up a mountain. Or running. If that's what you're into I guess.
  6. Go to concerts
    See above reasons for movies. But seriously, you'll meet great people and be carried away by everything. I went to my first Taylor Swift concert alone. Its perfection has never been eclipsed.
  7. Masturbate
    Self explanatory I think (hope?)
  8. Drive
    A good solo roadtrip does the soul a world of good.
  9. Meditate/think/dream
  10. Cook something new
    Makes it easier to experiment if you don't have a party of 8 arriving in 55 minutes. And if you make something delicious it's like a little birthday gift to yourself. 🎁
    Suggested by @evanp