Everyone else has been talking about Writer's block, I don't think I suffer from this, more from Idea Block. I'll know I want to do something but not have a good idea of exactly what to do. It's a sort of urge to be productive, or at least to avoid being bored.
This problem is compounded by my mind's habit of thinking most creatively when I'm either on a long walk or paddle, or in the shower. These being times when I'm far, far away from anywhere that I can use to move the idea into reality.
Often I'll carry a notebook with me, but that's just not practical when I go out on the river, so I try to hold things in my mind until I get back to civilisation (or out of the shower...). I find the best way to do this is to make myself very annoyed with whichever problem the idea should solve, as I can hold that small gem of bile in my head for days. This approach works fantastically when there actually is a problem to solve (software, physics) but falls over where the thought was a simple creative leap.
I think this means I'll never quite write a novel, but I quite like the time I can spend with friends, so maybe this is a good thing.
I also bought a great book today Vacuum bazookas, electric rainbow jelly and 27 other saturday science projects by N Downie. It contains 29 vaguely scientific, but still fun, things that can be constructed in a saturday afternoon. Each has a vague set of construction instructions, a collection of directions that could be taken to change what the project does, and a very good description of the science (including the all important mathematics) behind it. My favourite project is a nut cracker capable of exerting a few tons of force to a nut, which will crack the shell, but which then ceases motion, so that the nut itself isn't damaged.