Thought about lots of dares, but i think if you really want a permanent gap then you have to rip your anal muscules. And i don't think that you want to end up in a hospital with riped asshole.
but you should do bigger and bigger every time. The dare with ball and 20 pumps was very good. Try the toilet brush, or take a camera from the bike well put it all in your ass and pump it till pain. It can hold a lot of pressure so don't worry about blowing it up.