On occasion my pups have gone under a bar instead of over it. That might be a neat challenge, with some type of visual cue that means go under the bar instead of over, like maybe 1 or 2 bars set above the target bar. Maybe even have some type of extended crawl obstacle, forcing the dog to get slow and low.
Another thought is similar to a table, a box - but only be about 4 inches high. I use this in training. 2 x 4 s on edge in a square/rectangular frame with a plywood top with a rubber surface. I use a welcome mat made from recycled tires from the hw store cut to fit and glued down. Similar to a mark bucket but with more surface area and grip. Get on and get off. I'm not a fan of stopping or requiring a position on it, just get a paw on it. Several in a row could be neat, like hopscotch for dogs.
Water obstacles sound a bit messy, most of the fields we run in are dirt horse arenas. Add water to that and the dogs will get caked with mud. Maybe in a grass field, but even then the field would eventually get soupy.
Another thought is a gauntlet obstacle. Many clubs have pvc frames with some type of cloth mesh as fences. Put two in parallel next to each other to create a channel between them wide enough for a dog to pass. The dog travels through the channel. Go in one end, come out the other. Like a tunnel the dog's vision of its handler is blocked but the dog stays on the primary surface and their are no turns or tunnel bags. Lengths could vary, one big long one, or maybe several shorter ones in a zig zag or arc.
I think there could be more creative discriminations. Why is it only ever 2 obstacles? Why not 3: Tunnel, Dogwalk, Tunnel? Why is it always a contact and a tunnel - why not 2 tunnels or 3 tunnels - left tunnel arcs left, right tunnel arcs right, center tunnel goes straight. That would be a serious handling challenge. Imagine the precision required to steer the dog to the center of three tunnels.
Another interesting idea is to have a lifesize plush dog somewhere on the course, the dog just has to ignore it. No faults, just time loss if the running dog spends time checking it out.