Hey! I'm sorry that you're not feeling much great. It's been a rough year. I hope everyone around you is safe. Take care of you and your family. Here are few things you can maybe do to make yourself feel a bit better!

Listen to songs(Need a suggestion? Christmas Time Again!)
Read cozy comics.
Learn something you always wanted to learn but couldn't find time for. It can be a new language (speaking or programming), some new form of art, a new sport, anything that'll uplift your mood. I learnt basics of HTML, CSS and it feels very awesome when you learn something new, however small it maybe!
Don't want to do any of this? Doodle! Make repetative patterns, designs, shapes, basically anything your hand draws. It's a great exercise to soothen and calm your mind without needing any skills.
Watch movies. Choose non-sad movies for starting. Or simply start watching/rewatching F.R.I.E.N.D.S. or B99. From the personal experience, this is a very promising way of making yourself happy and motivated.
If you're away from your school/university campus due to the pandemic, go through old pictures of campus and your friends!(This might be counter-effective though. Avoid this if you were feeling sad because of nostalgia in the first place :/)
Last but the most useful, talk to a close friend. Go visit him/her if possible. Feel free to ping me if you don't know whom to talk to :)

For those who still don't feel much good,
you tried everything, but none of it works? Well, scroll down a bit, have a little gift for you...

Did you collect that cookie?
Good! Now eat it.
..
..
Done?
You feel good nowww? *4 y-o excited look.*

I know it was silly. You can't really eat that cookie :( *Starts walking away sadly*

Oh wait, are you smiling? ... Damn, That is the prettiest smile I've seen in a while!
Damn, did I just accidentally make you smile? Awwwww, it's even brighter now, isn't it?

Congratulations! Now you're tempararily happy!! Now go back and tell me that you're doing pretty good so that I can wish you a Merry Christmas!

And oh, ps, I need that smile when I wish you okaaaaay? ＼(^o^)／