Smiling - even if I don’t feel up to it I try to find something that genuinely makes me smile, even if it’s a daft cat video on YouTube or one of those cheesy uplifting life story things... anything that usually gets you smiling.
My usuals are (as well as the above mentioned):
Stand up comedy - something new if I’m feeling low as I don’t like to watch something I know because I know I’ll just be miserable about knowing what comes next
Disney movies - one of the few happy childhood memories I have is getting to watch a Disney animated movie
Horror movies (if you’re not scared by them) - I haven’t been scared by a horror movie in years so I generally find myself laughing at them or finding them so crap that they’re brilliant if that makes sense?
Apart from that what other PP have said, excercise in whatever shape or form it might take, try to get out and about at least once a day, learn a new skill or put an old one to practice if you haven’t done it in a while.
If I’m drained mentally (massive introvert so happens easily) I wait until my toddlers and DH are asleep and lose myself in a video game or some really good music.
Most of all try not to beat yourself up for being a bit grumpy at the moment, if you find it happens around this time most years maybe some Vit D supplements as a PP suggested?
I also find watching grumpy comedians great when I’m in a meh sort of mood - but that’s just my taste in humour I suppose :)