If your one of my players bugger off.
So one PC is going to appear through the lake (which is a portal to neighboring planes), the other is that last survivor of a party coming to raid the ruins, and had some means of bypassing the site guardian.
My basic set up is going to be a mirror still lake that reflects the sky so well that it give the same impression as standing on the edge of a cliff, or looking up and the night sky. The lake is enchanted, and I think that one of the obstacles will be a will test to avoid falling in and becoming cold & wet. The lake is encircled by a few remaining standing stones covered in worn magical glyphs… not sure what they do, but they should probably have the potential to do something. There’s some a neck of rocs here, and they will likely attack the party and steal their food or some such. The eggs could be loot if they find the nest and drive off the parent (s). The whole site is guarded by a golem of sorts which won’t disturb them until the party tries to leave. There’s a pack of dire wolves on the mountain side as a last encounter.
There’s the ruins of a small tower, I haven’t decided whats in there yet. The site will of course have some loot. A broken enchanted orrery for one, and I think the bodies of past adventures.