Manual activities that they can get involved in: make play dough, beads on a string, decorating paper planes, dress up, lego, build a track for toy cars.
If you can fill a bath and let them play in it. Mine could splash happily for half an hour.
With some effort you can play pass the parcel, or assign to each of them a sound to make and you are the conductor like in a band.
Darken a room and give out flashlights.
Build a fort/castle.
We've raced cars around, made paper airplanes, the 3 yr old made me draw everything she could think of. I like these ideas. Some won't work with the 18 month old but you've given me ideas for her too. Thank you!
18 months is challenging... It will get messy, but if you have painting colours, put some on a paper plate and then let her "paint" with different things: toy car, balls, pasta...
On youtube there are videos of dancing fruit... For a 10 minutes break they are great, especially if you can make them dance as well.
We have three: ages 1, 3, and 6. Our go-to move is to get them outside. In the driveway or yard, going for a hike, to a playground, etc. Anything outside keeps them sane. The longer they are inside the more insane everyone becomes.