EliptiGO: Played for short spins in and around a local bike shop. Interesting motion and handling was tricky at the start, especially at low speeds. Can get the hang of it without much time, and is better once up to normal speeds most people would use. Not something I’d even consider owning, but a fun thing to try at least.
Swing Bike: DIY model based upon a super cheap MTB, built by a friend. The nickname “Something Terrible” is apt and it is a scary affair no matter how much time I’ve been on it (3-4 rides over a decade). This one was a quick and dirty build, and the slight misalignment of the pivots is the challenge, but also the character. Super fun when you get it figured out, but you still have to treat it with respect or it can bite and toss you quick 
High Wheeler: Got a chance to ride one once, and it was also scary. Something about being up so high, on a heavy bike, and doing it in more limited space parking lot than you’d really like. Just enough to have a taste that you’d really have to want to ride a bike when these were the definition, to get over the unique ride and feel. Pretty fun for the moments you feel in control, but starting and stopping are the points of nerves for me.
Double-decker Tall Bike: DIY from a different builder. Similar challenge to mount like the high wheeler (both at the same event), but more like a “bike” once you are moving. Gets stable at speed but the slow stuff is equally scary, even with my decent balance. Tall vs High Wheeler, the Tall Bike would be my pick if I had to pick one.
Reverse-Steering Bike: More DIY and something I never really nailed in the 5-10 minutes I had with it. Super crazy and mind bending as you can see in any related videos. Sort of like puzzle you can’t solve, but want to try anyway.
Retro-Direct Drivetrain Bike: Super fun toy considering it was built on an old Nishiki elevated chainstay MTB that I drooled over back in the day. One speed pedaling forward, a different speed pedaling backwards. I forget the direction for which was the Hard vs Easy gear, but it was pretty intuitive to use in a short time. This is one I’d have from the lot above, for a pub crawl toy.
Unicycle: Not sure this counts in the category, but I got one for my birthday after I got into bike trials. Great challenge to conquer and I still pull it out every couple of years to relearn. Also the source of a Top 10 wreck in my riding career. Learning in the garage, got about 2 pedals into an attempt, balance rearward so step off with one foot, but the Uni continues forward… while the forward pedal trips the same foot I’m trying to catch myself with… BAM!!! on my tailbone faster than I could imagine. Out of commission for a week to recover from that single misstep 
