Here's a fun project from The Toddler's Busy Book. Very simple!
Paint on some bubble wrap (taped to a table), and press paper onto it to make a print!
We used cheap acrylic (acrylics are non-toxic) craft paint. It's easy, but definitely can be messy. I only had BIG bubble wrap, so that's what we used. I think it would work better with smaller bubbles. But the result was pretty neat:
This lasted about 10 minutes with my 19.5 month old before she told me she was "all done". She had very little interest in the result (beyond wanting to paint on it more... she will paint on a paper until it is a sopping wad of mud-coloured fibre if given the opportunity). There was probably almost an equivalent time in prep and cleanup. But it was fun!