I did a playtime tummy mat for my granddaughter a while ago by using 15" blocks and appliquing simple object shapes onto each one - apple, sheep, turtle, pig etc - and used all different types of textures for her to feel and scrunch up.
It was a big hit with her and her mummy. It was easy to wash in a washing machine also - very important with all that dribble or worse.
There are a lot of ideas so have fun and show us what you make please.