Oobleck in the sensory bin!
The texture of Oobleck, (corn starch and water), feels hard when you first pick it up, but then the Oobleck runs through your fingers giving it a liquid feel.
Oobleck looks super messy, but honestly is so easy to clean up! With a little water it will wipe right up, or leave it to dry and it can be easily swept or vacuumed.