So we made water balloons.
Just filled up some regular balloons with water and they were perfect for throwing, bouncing, rolling, bobbing - they even withstood a little biting!
The trick was to leave extra space for the water to move around - too full and they'd burst! The little ones were our favs so we ended up making two more. (They were easy to hold and not too heavy for Max).
Squishy, blobby, cheap fun!