Depends on how big a splurge I need.
If I'm just a little down, I'll go to a bookstore and get myself a French soda at the cafe (that's an old fashioned soda with a shot of half and half to make it "French"). A half hour of unwinding there is pretty rejuvenating. Alternately, I get a pair of slippers that makes me feel pretty. $5 tops, since I get them at discounters. I have a collection of them now, which is odd, but it makes me feel taken care of, so who am I to question? (My husband started the slipper thing.)
If I'm really down, we set a $20 limit and go to a $ store, dollar spot in Target, or similar place, and I throw together a care package for myself. Recently I put one together with a book ($1), a box of really good and unusual chocolate candy ($2), a silly movie (Mamma Mia, $5), a coloring book ($1), facial kit ($2), and I forget what else. You can take $20 a ways at the right store.