I find wearing clothes which you like (and especially those you've made yourself) fills you with pride and cheers you up - even on a gray, cold day. They say you can do 'dopamine dressing' which means you give yourself joy by wearing certain clothes. For me, those items are colorful and I feel comfortable wearing them.
As far as the outdoor activity I prefer in the autumn, it is taking long, often meandering walks. I think the darker days are a good time to rediscover a local park or take a stroll with a friend. You don't have to fight the heat nor fight off the freezing cold of winter and you can get lost, wander around and take your time.
I wish you a lovely time this fall during sweater weather!