I've modded using the GECK for quite some time and I have noticed some weird things you can do with it. I'd like to know some of the things you've tried that were at least partially successful. Don't post stuff that was a complete failure. That is no fun. But do post weird stuff that you've encountered.

I'll start with this...

On a lark, I tried making a floating island way, way up, above Megaton. It was so high you couldn't really see it from the ground unless you already knew where to look. I made a little island surrounded by water on three sides. You got to it via a standard teleport door. You could swim in the water and have a great time on the island with no ill effects. Even if you swam through the bottom of the water object, the game allowed you to continue with the swimming animation all the way back down to the ground. You didn't fall. Once you touched the ground, you reverted back to walking. The only drawback was that once you were below the water object, hovering in the air, you could only swim level or down, but not up. Another cool aspect was when jumping off the island where there was no water. Then the player would sky-dive for quite a long time before hitting the ground and dying. But in the meantime, there were plenty of great views/screen shots of the Capitol Wasteland.