Ha! The things they do…....amazes me. I know your pain, mine escaped several times from his x-pen......took forever for me to figure out how to keep him in there.
Nosework is a fun way to exercise them ; even 6 rounds can tire them out. It's cheap, and you don't have to go outside necessarily. I wish I had started my dog when he was a wee pup, BUT once they get to an 'advanced level' you can hide things for long periods of time and they have the persistence to keep going until they find the scent. The classes are nice if you want to figure out how to do things the 'proper' way, but if you are strapped for time and money they have lots of free tips online. Just start out slow and work your way up.
There's a few videos of basenjis escaping, on youtube, that are fun to watch. This site is great, lots of good ideas being passed around.