Some things you can do on a rainy day are sleep, read books, watch youtube, browse the internet or watch a good movie or movies.
I like to read books, take naps and sometimes curl up on my couch under a blanket and just watch endless movies on Netflix.
If I get bored of watching Movies I'll fall asleep on the couch.
Rainy days seem to put me to sleep more than sunny days so those rainy days are good days for me to catch up on my sleeping.
If you have toddlers or children take them outside to play in the mud or jump in mud puddles like I did as a kid.