Preparing for your few computer hours sounds like a good idea.

I would also try to prioritize and polish your work. What should be done first, what should be done second etc.

Polishing is about not adding new features, it is about ending the development cycle.

You can add new features that you worked out for the 60-80 remaining hours, you can also stop earlier, and start polishing what you have.

Does everything work as intended, even if you do weird stuff, walking back, forgetting to pick up something, can you always go back to pickup the missing item?

You know the game inside out, so you take the optimal route through the game always, let a friend play who doesn't know the game, and watch, but do not give directions (maybe even not say anything). You'll see he makes different moves, does things in other orders, etc. All that should work.

Are the graphics right, does the high score work, is the explanation what to do clear, all the little things that need a little touch at the end.

A lot of game play is not about how many features you have, it's also about how well you present the things that you have, in game play, graphics, sound, etc.

If you're decent at coding, you can code your easier code on your smartphone, then email that to yourself. Use your computer-time for refining your code, debugging, etc. Then use your remaining non-computer time for art assets, etc.

This is what I did while I had a job that gave me alot of freetime, but only a few hours a day in which to code. I used my downtime at work to write up simpler scripts, plan my agenda, and write 'to-do' lists.

It will be more difficult to code without a compiler to check for errors or a copy of your existing code to reference, but not impossible. It will also force you to write more modular code that you can simply plug-in to your existing code, rather than a spaghetti-string tangled up mess.

Basically, use your no-computer-time to make your computer-time more effecient. Create some kind of shorthand for when you don't know something, and fix it when you see the code.

If you're using an engine or library, it would be helpfull to keep a copy of the scripting reference on your phone.

Get a cheap or free PC and put a free operating system on. If your parents don't allow it put it up at a friends house.

The idea of planning your code for 6 hours and then code it in 2 hours will not make you code any faster.

Rather plan it for 10 minutes and then code for 8 hours. But sure in the rest of your free time read a lot of documentation.

If you have access to the internet you can use a online IDE. Honestly if you have internet you can do almost any thing you want.

Planing is good way to use time, most of game design IS design, so no point in doing nothing.

Working on art is always a option, draw stuff and load it onto the PC when you have time. If you don't have a scanner take photos using your phone.

Go do other stuff, becoming a great designer means you will need a lot of experiences, to turn into game ideas.

I would rather try to find a solution to get more hours behind a PC. If your parents do not allow it then explain what you are doing instead of playing games and involve them in your project. This can give good motivation and feedback for you as well. But I don't know the exact reason why you can only spent a view hours each day. If you don't own your own PC then build one yourself, if you are lucky you can get a pretty good one for just $25. Plenty of people that can help you with it on hardware forums.

Making notes and looking up sources is a good idea for your spare time but you cannot try it out and for a lot of things you need to go back and forth between documentation and coding.

Anyway, 3 hours multiplied by 30 days is still 90 hours. Creating a full (yet simple) game in 90 hours is totally doable if you have some experience. So finishing up half a game should be possible, depending on the game of course. For a competition this long most people can only commit about 3 h/day = 21 h/week since they probably have a full time job.

Hey,
A little bit information of me before we go further:
I'm 15 and in high school. It's now summer and as stated in the title, I only have a few hours (2 - 3 hours, that's not enough) of access to a computer but a lot of free time. I also have a smartphone which gives me internet access and allows me to post this.

Here's the problem:
I have a small half finished game which is also my first one I will ever finish and I want to join a game competition with it whose deadline is on the 5th of August. So I have roughly 30 days to finish the game. The parts still missing from the game are the most fun features which distinguish my game from the others of the same kind, and the level design.

So this is my solution:
Use my free time to come up it all the ideas, problems I have and will have and the solutions to them. And take note all of them into a notebook or into my smartphone. All the notes I take must be elaborated so all the time I have with the computer should be only coding with the support of the notes not thinking as it's easier for me to think when I'm away from the computer.

TL;DR:
Problem: Have small game to be finished in 30 days for a competition.
Solution: Use free time to come up with ideas, solutions and take notes.

Do you guys have any thoughts on my solution (is it a good idea and I'd better start now) or any advice, tips or maybe some suggestions or apps for my note taking?

What language are you using?

I would recommend the following.

-Pseudo code on paper.

-Use Runnable via mobile.

http://code.runnable.com/

Although they don't support .net on the mobile first version of this website, but I hope this helps you out. You do need to create an account for this.