Clean up. It takes a certain amount of focus, but also is very rewarding when you finish. It is good to have a sense of control of your surroundings, but I also find I get quite proud of myself when I can maintain a nice clean space.
Give someone a gift. Someone you rrreeeaaaalllllly love and care about. Get them something they aren’t expecting and that you know will put a smile on their face.
Buy yourself flowers. Treat yourself for once! Pick the prettiest ones in the store and plant them right in the middle of your dining table so you can see all the bright happy colors.
Take a walk. And really notice everything about your surroundings. When you are more attentive like this the world looks so much more beautiful.
Make a schedule. If you have a seemingly impossible amount to do, break it down into smaller pieces and stick to it. You can do it.
Cook some really complicated fancy food. Take a lot of time and care putting it together and make sure it looks stunning on the plate. Heck, take a photo and share it with all your friends so you can be as proud as possible of your work.
Do yoga. Or any sort of slower exercise requiring a lot of focus. Exercise releases happy endorphins, and yoga is calming. This is a great combination for turning up your mood.
Hope this list is helpful. What do you do when you are feeling down to cheer yourself up?