Interviews

Largely it's up to you. Some candidates travel the day before their interview and stay in college the night before, and others even stay the night after their interview and travel home the next day. So it's dependent on how far you have to travel, and whether you'd like to sleep the night before/after (I stayed over the night before, partly because I was travelling via quite a complex train route and I wanted to eliminate the stress of late trains/missed connections which could've made me miss my interviews, and partly because I thought that in case I didn't get an offer, well, I could always at least say I'd stayed in a Cambridge college!)

As for tours, this varies by college. My guess would be there won't be much on offer: you're there to be interviewed, not to have an open day. Schedules tend to be quite busy; you'll likely have multiple interviews and potentially a test, so you probably won't have all that much time sitting around twiddling your thumbs. If you get an offer there'll be plenty of chances later to visit the college for open days and tours, so for now just worry about getting in