To make an unconventional ball pit like ours just fill up a kiddie ball outside with all different types of balls that you have on hand. O balls, beanie balls, interactive balls, bumpy balls, beach balls, etc. The more variety the better!
My daughters was really eager to get in and explore all the balls. She thought it was hilarious.
She played in there for a bit, exploring all the balls. This is a great time to add to vocabulary if you're describing the balls to your little one.
After playing in the pool for a while we turned it into a more active gross motor game. I was sitting on her bench about 10 feet away from the pool for this. She would get one ball and run it to me. Then run and get another, and so forth. This was a very simple way to work in some great exercise for the day.