Reading the "Campsite at Drumcree" thread(s) on Mudcat helped a bit when I was in my recent pit of despair. And the "Humor Us" threads have enough daft jokes to curl up the corners of your mouth. (The joke perma-thread is here.)

Listening to Tom Lehrer's "Vatican Rag" makes me laugh out loud every time.

Rock climbing at the local climbing wall. I've found that climbing and climbing til I fall off (safely) works wonders to get adrenalin flowing and tires me out. Sense of achievement plus sense of exhaustion - mental and physical focus so you can't think of anything else, even if you want to.

Are you near the sea? That's usually a good spirit-lifter. The smell and the wind and the water and the sound.

Buy some nice food, spend time cooking a good meal, enjoy it with a great wine.