I know, it's a weird one, right? But the thing is, this really helps me. I tend to bundle this in with other things, like running or biking, but for me just being outside enormously lifts my mood.
Now, I am slightly fussy about the outdoors I choose. I like green and leafy. I like to be near water. I like it to be pretty and scenic. Being outdoors in a car park is not where it's at here. And if I am in a pretty location, then I find that the weather is irrelevant. In fact, I quite like a bit of light drizzle.
However, the main reason that being outdoors works for me is because I find it the easiest way to practice mindfulness. I make a point of looking around for pretty views, for squirrels, rabbits, birds, ducks and so on. But more than that, I love finding a patch of sunlight coming through the trees in a glorious way, seeing sunlight reflecting off still water or watching the rain cause concentric circles ripple through it. I actively try to find the beauty in small things, to slow myself down and actually look at the world and just take it in.
I have also been known, when my anxiety had unexpectedly reached a crescendo at the worst possible time, to walk bare foot in circles on a lawn while sipping peppermint tea. It really helps!