Every programmer that has a project in mind should try this: Put away 3 weeks of focus time in a cabin, away from work and family, gather every book or document you need and cut off the Internet. Use a dumb phone if you can live with it. See how far you can go. Just make sure it is something that you already put a lot of thoughts and a bit of code into it.
After thinking more thoroughly about the idea, I believe low level projects that rely on as few external libraries as possible are the best ones to try the idea out. If your project relies on piles of 3rd party libraries, you are stuck if you have an issue but without the Internet to help you figure it out. Ken picked the right project too.
I think this is key. If you already have the architecture worked out in your head, then it's just smashing away at they keyboard. Once you have a 3rd party library, you can spend most of your time fighting with and learning about that.
What I have in mind are embedded projects -- you are probably going to use it even when you are the only user. So that fixes the motivation issue. You probably have a clean cut objective so that clicks the other checkbox. You need to bring a dev board, a bunch of breadboards and electronics components to the cabin, but that doesn't take a lot of spaces. You need the specifications of the dev board and of the components used in the project, but those are just pdf files anyway. You need some C best practices? There must be a pdf for that. You can do a bit of experimental coding before you leave for the cabin, to make sure the idea is solid, feasible and the toolchain works. The preparations give you a wired up breadboard and maybe a few hundred lines of C code. That's all you need to complete the project in 3 weeks.
Game programming, modding and mapping come into my mind, too. They are fun, clean cut and well defined. The thing is you might need the Internet to check documents or algorithms from time to time. But it is a lot better to cut off Internet completely. I think they fit if you are well into them already -- and then you boost them up working 3 weeks in a cabin.
There must be other lower level projects that fit the bill. I'm NOT even a good, ordinary programmer, so the choices are few.