Lifelong single here so I have lots of practice with this. Pick and choose as applies.
1.Manicure, pedicure, massage day
2.Take a trip (Day, weekend, whatever)
3.Buy yourself a fun splurge gift or more than one. Doesn't have to be expensive although that's okay too. Just something special.(Nice yarn if you knit, perfume, a book). Wrap it nicely. Don't cheat and open before.
4.Order something online timed to arrive around your birthday. Bonus if you forget about it and it's a surprise!
5. Visits with friends or family. Phone calls with loved ones you don't see a lot. Any of these can be done by yourself or with company mostly.
6. Shopping trip for a nice outfit.
7. Take a class on something you want to learn.
8. Concerts, theatre, or a movie out. Get the popcorn!
9. If you stay in, plan it out like a date with yourself. A nice meal, entertainment, music you like, comfortable clothes, pets, hobbies, bubble bath.
10. Something, anything, that makes you happy. You deserve it.