A Monty T219 mod trials bike is how I keep my bicycle balance up to par.
This one is pretty old.....I bought it from it's second owner way back in the early nineties. It's held up nicely though!! The only real issue it's ever had is that I replaced the headset a few years ago and, oddly enough, this is a Mexican bicycle but it has a French headset.
I actually sold the bike once and it was given back to me years later.....Then I gave it away to someone in need and it made it's way back to me even more years later. It's like when you set free the one you love to see if they come back. Hard to believe I first jumped onto this bike twenty years ago!
You don't need to do anything "extreme" to gain skill from a trials bike. Just jump on and do 5 minute balance/riding stints around the house without allowing your feet to touch the ground. It's helpful for building your strength, balance, and reflexes.