In my garden I just have a glass bowl on a slab of sandstone and decorative rocks around it and I'm using it currently as a birdbath but I was wondering is there a really easy way to make a small waterfall into the bowl?
There is indeed. You can find small pumps often at Hobby Lobby or similar craft type store. You will need somewhere to plug it in, and a way to hide the cord after you put the pump in.
Your biggest problem is going to be keeping the water from splashing out. Maybe water can just flow out and over the bowl.