Rainy days may not be welcome, but we can still enjoy them.
It might be the kind of day when you don't want to open the front door at all. If you have kids, this is a great excuse to facilitate some great games and activities, and then take a back seat (you are meant to be relaxing after all).
You could pitch tents in the front room and have a picnic inside. You could all collaborate on making a highly individual cake or cookies for someone special, for an upcoming birthday or just a surprise gift. There are endless ideas online for involving children in baking simple treats. You can also let them organise the cleaning-up responsibilities...
For older children there are big family board games, and many fun word games that involve everyone (for example, Cheddar Gorge: you all invent a story together, one word at a time - everyone says a word in turn. Hilarious).
In some ways, rain gives you permission to knock off for the day. You could have lunch somewhere nice and watch the drops run down the window; take the whole family to the cinema or go to the pub for a laid-back lunch.