I study Computer Science (G400), so Politics is completely different to my main subject. As part of CompSci its compulsory for me to do 'Introduction to Programming', which other people can choose to do as an elective - so if you're interested in Computers, then it's definitely worth looking at. You learn Java, and it's reasonably easy. The lecturer (Stefan) is also really friendly and no past experience is really required as everyone (even CS students) has never done Java before.
I don't think they have any effect upon your degree, but if you do something relevant you can always include it on your CV to show you have experience. It also gives you a backup plan if you don't like your degree and want to switch. Theres bound to be something interesting to pick.
It's also good to make some other friends away from your main degree and away from your college.
There is a timetable checker website, but it sometimes shows clashes when there aren't any, so it's best to check with the departments. Even without timetable clashes there are sometimes difficult to get from the Science Site to Old Elvet instantly
I suppose you can do stuff on the other campus. There is a free bus (X11) which wasn't terribly frequent. It takes 30-40 mins to get there, so i'm not sure it would be worth it just for a 1 hour lecture/tutorial. Speaking from experience, the buses are older than we are and not very well maintained. I'm almost certain that the exhaust is pumped back inside the bus as you can see a smoggy internal haze when the sun shines through the windows and you can't breathe properly. Don't let me put you off though!
Alex