So I'm on holiday at the moment in America visiting family, and I don't have access to a guitar. I don't know why I didn't think of this earlier, but what sort of music theory should I have a go at learning in my free time?

I have a very basic knowledge of theory, and am starting a Music Diploma College course in september. What sort of theory should I begin to learn so that I am not so far behind everyone else, remembering that I can't actually play guitar for another 3 weeks.

Website and links would be appreciated.

Do not learn modes. I have a very strong perception of music theory and modes still confuse me - can i ask how you got into college to do a diploma in music with barely and theoretical knowledge? I'd cram if i was you, because you're gonna make yourself look a mug if you don't know what you're talking about. Absolute essentials - major scale construction, chord construction, cadences...also a basic knowledge of reading the stave would be good.
Modes? What has that guy been smoking?
Learn all about the major scale. Write it out for C major and learn how triads are built.
Do some basic reading on counterpoint and some on harmony. Also, ear training is incredibly important so try to learn the intervals at least.
a good start
Inhuman evil take down!
All I neeeded to do for the audition was play two pieces of music, and that was pretty much it. They didn't ask me about any theory. I'm 16 and it's college in the UK not American College.

I will look into the scale stuff and chord construction. I can work out sheet music but i'm not completely fluent, so i'll recap that as well.