I do mean like ingame things. Weapons would be...interesting to do. Unless they were just in schematics. But I mean anything really ingame that is created by you.
*I'm not looking to create content and add it to the game, I want to make things ingame
There is not much of a distinction. Anything you can create in-game, must first be content added; built by a modder or a developer, and put in the game via the GECK. The engine is not designed to extrapolate improvised objects.
Here is a weapon mod that provides an alien Steam-Punk rifle, that the player can upgrade, by using parts from a plasma rifle.
http://www.fallout3nexus.com/downloads/file.php?id=7751
Here is a Youtube video of the mod in action...
http://www.youtube.com/watch?v=Al3qhaMHn6A
http://www.youtube.com/watch?v=eTzfyevGNtI