I explored every inch of the campus, slept in weird places, ask me questions
While I studied at Lancaster uni, I used to get very restless at night and ended up exploring the whole damn thing in the dead of night, I also had cause to sleep, and temporarily live on campus (not in accomodation). I no longer live in the city, and I feel like I have a lot of essentially useless knowledge that might be useful to current students.
If you have any questions about how to access certain areas or the best places to go at various times of the day/year to access certain types of facilities, drop me a reply (or a DM if you want to keep it quiet) and I'll help as best I can. My knowledge is a little dated, as I moved 9 years ago, but I imagine a lot of it is evergreen - except guidance on how to get on top of the spine (classrooms above Furness, often accessible 24/7, with windows that opened out onto the spine) since it's all glass now.
Coolest thing I saw that no longer exists - when they were refurbishing Ash House, it was out of commission for a year. The doors were often left propped open by builders, and I got to explore the flats, entierly abandoned, and undergoing refurbishment - arcing around the hals, you could essentially walk from dated to very modern accomodation in just a few steps. Automatic hallway lights did give me away to security, but they also gave away their location, so it felt a bit like a game of pac man.
The best college is Pendle - gives you access to the TV room with your student card, which is a godsend if you need to go to campus during the summer holidays for resits. Comfy, lots of sockets, and nobody is there in summer. Also just a cool space generally. Be aware that there is a cleaners closet in there, and cleaners will come to get supplies from it, usually at around 9am during summer hols (check cleaning rotas on nearby areas), so if you're asleep in there, be awake before they come.
There are cooler spaces in other colleges, of course, but most of them can easily be accessed by anybody in any college. Pendle gets you the most bang for your buck in terms of unique benefits.
You can sleep in the toilets near the LZ if you need to (lots of them so nobody will notice it being locked) provided you pay attention to the cleaning rotas on the back of the door - but in my experience you're far better off just sleeping in a dark nook somewhere (LZ study rooms are great for this, just lay on the floor behind the seats, o nthe size not facing into the central area - you're invisible). If you sleep head down arms folded at a seat, even if somebody does wake you (unlikely), they'll just assume you fell asleep studying - and you won't attract any suspicion. If you absolutely must sleep in a toilet and do not want to be disturbed at all, the one on the walkway behind Juicafe, Greggs etc (access it by walking around the santander) has one that is just used by the staff at those outlets during opening hours.
Best place to roam at night is Furness. Lots of stairwells that remain unlocked (or can't be locked) - upstairs office areas have plenty of sockets and tables, and no footfall. Great for a quiet late night study session, and there are even staff facilities (fridge, microwave, kettle, toaster) so you can take food and cook it at your leisure. Do not steal other people's food, obviously, and clean up after yourself.
Safe exploring, don't break the law, don't ask me to help you break the law. For clarity, nothing I did was illegal. Trespass at a push, but that's a civil matter, and ultimately I was just a student on campus in areas that I could access freely either through opens doors or that opened with my student card. I never got into any issues with the university, even when found, just politely told to go elsewhere occasionally.