Inflatable ball pools are great because they can be filled with balls in the Winter and used inside but in the Summer you can fill them with water and use outside. Check out my top choice of inflatable ball pool for babies below.
I have just bought my baby a ball pool for Christmas. He was 6 months at Christmas so old enough to sit in a ball pit and play with all the balls. Before making my purchase, I did some research about what makes a good ball pool for a baby and this article details my research to help you decide which ball pit / pool is best for you.