Swipe art is good for older people with limited hand mobility. Put dots of paint on the paper or canvas then let the participant swipe with different tools: small heavy cardboard, sponge, flexible brush, the butt of a closed hand, etc.
Fun art project with the kids! Easy abstract painting that anyone can do! Just blob the paint on the canvas in random spots and blend! (would be interesting how students could make different pictures w/ the same colors and same blob positions)