When mine were that age I gave them a few blobs of paint on a baking sheet. They would smear it around for quite a while, either with a brush or more usually their fingers. They usually ended up ripping the paper when it got soggy so this worked out really well.
Peeling and sticking sticker was (and still is) a favorite too.
Threading cherios or some similar cereal is good too, and it doesn't matter if they eat the "beads"
I bought a bag of small craft pom poms which have kept DS busy for quite a while. He likes to scoop them with toy diggers etc, sort them into an egg box and just generally move them form one place to another
DD (4) likes to layer them in a clear jar.
On day we may even stick them to paper.