I have a bathtub/shower combination that we let our kids "fingerpaint" when they were little. We would put them in their swimming suits and give them the paints. They would make "art" on the walls. I took pictures of them and their art. Then I would spray off the wall and let them start over. They did this for HOURS. I would use this time to clean out the bathroom cabinets & drawers. We would sing, laugh and tell jokes.
Another thing I liked to do was get old socks, decorate them as hand puppets with yarn, buttons, markers, fabric, etc. Then we would pull out the sofa & get behind it to use it as our "stage". We would stick our puppet hands up over our head and make them talk. What a hoot.
The tent thing was cool, but sometimes we would have an indoor picnic. I liked letting the little ones spread the peanut butter and jelly. The older ones would peel carrots and cut fruit. We would pretend we were outside. When the storm passed, we went outside for puddle jumping. Just remember to put towels outside with a big tub for wet clothes. Strip down before everyone comes in and go shower or bathe.
We used rainy days to start planters, usually a flower (marigolds work great) that they could transplant outside when it got bigger. My kids seemed to enjoy the science project kind of stuff.
We would read a lot of the same books, so I would have my youngest ones "read" to me. They weren't really reading, but re-telling the story by looking at the pictures and remembering when it was read to them. It was funny to hear some of their translations.
Crafts, crafts, crafts. Macaroni necklaces.... glue macaroni onto paper for art, home make playdough... coloring, scissors, glue, glitter....
I had a box that I saved things like that in for rainy days.
Good luck and HAVE FUN. Just be a big kid and you won't have any trouble playing....
Before you know it, they will be in college or grown and move away.