When I want to improve my mood, I always look first for something in my complete control. Purposeful action is the best one. I can decide on a purpose and I can take actions toward that purpose. It doesn't depend on circumstances or the moods of others. Doing something purposeful is a self-reliant way to reliably raise your mood.
Think of one small goal you really want. And it's really important you think of something you want. You could do things you should do all day long, completing task after task, but if there's no juice in it, all that accomplishment won't raise your mood. (Well, that's not entirely true. If you have something worrying you or something you're dreading, and you get it done, it can be a relief.) But for the real enjoyment, you need: 1) something you want to accomplish, that 2) you enjoy accomplishing.
If all you're doing is the drudgery, thinking that "some day" you'll get around to doing the stuff you love, you can really improve your ongoing mood to add even a little of something you really want to accomplish.
So that's your assignment. Do a little of your joyful purpose today, or if the day is almost done, then start tomorrow. Think of something you really want to do that you really like to do, and get a little of it done.
And try to do a little every day. It will add a lot of fine feeling for a small amount of effort, and that good mood will linger. You'll get the benefit of anticipating it, enjoying it while it's happening, and feeling the lingering satisfaction afterwards.
If you want to get in shape, and you love to exercise, work some into your day today. Even a little bit. For me, I love to read. So I try to work in at least a little reading every day, and it boosts my mood even into the next day. I think about what I learned and I try things out, experimenting with the ideas. I share things with others, and it makes me happy.
What is it for you? Do just a little today.
If you would like to read more about this, here are two good (and short) articles:
The Ocelot Blues
A Lasting State Of Feeling Great
Adam Khan is the author of Self-Help Stuff That Works and Cultivating Fire: How to Keep Your Motivation White Hot.