S.T.
My kids used to have a great time making forts out of blankets, chairs, couch, folding tables, etc. It made the house messy and crowded but they had a great time and would go under their forts and read books, play with legos, etc. Even the cats liked them. between the set up, play and clean up it would consume hours.
Do a scavenger hunt around the house - make a list of hints and hide trinkets. sweets, even old toys he's forgotten. At every step he finds a new clue to the next trinket. That takes a little time to set up but they have a ball with it.
Macaroni decorations were big when we were kids. Use uncooked macaroni in different shapes and glue them to wreath shaped cardboard, styrofoam cones, etc and then paint & glitter. You can use the same concept with pompoms, cotton balls - or with gumdrops (except you'll need straight pins so it's not as easy on little fingers).
Birdfeeders - string some yarn around a pinecone for hanging. Spread peanut butter on the pincecone and roll in bird seed. Hang it outside the window and watch the bird show.
Good luck and have fun!