You can easily pick up some nice pebbles or rocks on a trip to the beach or on your walks out and about.
All you then need is some acrylic paint and a little imagination (or google some simple designs online)
My son made smiley faces and I made him some little bumble bee and ladybird stones. I also saw some brilliant food ones the other day where someone had painted them to be part of a veg patch as carrots, potatoes and cabbage stones. There are so many possibilities with these.
Once you have painted them you can hide them in your garden orna local park and invite your friends to find them. There are also lots of rock painting groups on fb you can join to show any other rocks you have found and encourage people to find yours.