1) Try doing some activities that are exactly the thing you wouldn't normally do. Like... I don't know... maybe go to a museum, join some kind of club or social group you'd normally never consider.
2) Get a cat or dog. Or a chimpanzee.
(Well... probably best to stick with just a cat or dog. But then again, a chimpanzee might be distracting enough!) So ya, it's better to go through life with a great friend you love. And there's many types of great friends we can share our lives with, and sometimes they're not even human, but rather a pet.
3) Pick a project or hobby that you can work slowly on, over time. Write a novel? Restore a car? Build a computer? A robot with lasers? Collect stamps?
4) Go for walks. Physical movement can help prime our neural networks, and enhance mood.
5) If you have access to a car, pick a random spot on a map... maybe somewhere out in the country side by a river or lake, and just drive there. (Don't do google satellite or street view: the point is to actually go there and discover it for yourself.)
Listen to fun music on the drive there.
Some excursions will allow you to find new and awesome things.
Some excursions will be totally boring and pretty much a waste of your driving time--you can't win them all--but hey at least you got to get out a bit, and listen to some tunes!
Try going down a few random roads as well, seeing what else you may discover along the way.
6) Watch some cooking videos online and explore new foods and dishes. As you attempt them, some will flop and end up looking more like dog food, some will be a big hit!
7) Speaking of food, small changes to diet can sometimes help greatly. Maybe try taking a multivitamin 2 or 3 times per week, in case you might be missing a micronutrient. Also add a few interesting yet healthy sides to your meals.
Maybe add a little touch of fruit for desserts. (Note: for some reason fruit ALWAYS looks way better and more fun to eat, if you slice it semi decently, and put it in a bowl, rather than just plunking down the whole fruit in front of you!
For example, I would buy apples, but then discovered that if I simply sliced the apple into quarters or eights I suddenly wanted to eat them! Same with oranges. Presentation is a big factor for the human mind.
8) If your place is messy and grungy, clean it up gradually! You don't have to do everything in one day, or even one month. Maybe find yourself a nice new piece of furniture you've always wanted... like maybe an antique cabinet... or a sofa or table from Ikea... or cool glowing LED accent lights... something fun you wanted.
For this one depending upon your budget you might have to slowly save up for it, but that gives you something to look forward to.
9) Finally... some people have biochemical issues that can lead to symptoms of depressions. So at some point you might have to see the doctor or a pro if you think it's getting bad.
I mean, it's perfectly NORMAL to feel sad or depressed on some days.
Sometimes people feel suddenly gloomy, sad, depressed just because its cloudy, cold and raining outside on a Saturday!
So we're all humans and you'll always have your down days.
But if they get to be too much then you may have to see someone.