Your son sounds like my kids. They had to be doing something all the time.
When my kids were 18 months old we did a lot of crafts involving cotton balls. They loved gluing them to things. At Christmas time I would print off pictures of santa and have them glue on the beard. We put snow on pictures and snow men, fluff on bunnies, etc. It was cheap and easy and if I put down the glue, not too messy.
ANd I learned this from a momma on here. A cookie sheet makes magnet fun portable. My kids both loved that, my 3 yo still does.
A piece of pvc pipe and hot wheels STiLL entertains my kids for hours. Set up a ramp and let him slide the cars down it. When he gets older, he'll start stacking boxes and making contraptions with the ramp.
Boxes and tunnels. (Ikea has a great price on a toy tunnel).
Sometimes we made reading "nests" on the floor. Pillows and blankets in a circle. My kids would pretend they were birds and we'd read stories.
Play doh. I know some moms hate it, but I love it.
Beach ball. It's great for indoor and outdoor fun. Pretty safe to use inside.
The bath tub. Any time my kids got really bored or really cranky, we'd go play in the tub. Instant attitude adjustment. :)
Hammer and nails. Obviously, not the real deal. My son
was two when we did this one. He has always been obsessed with tools. So I would let him hammer his pretend oversize nails into some styrofoam sheets we had. He LOVED this.
Neighborhood walks are great.