Hi CatMandu, it's always a pleasure to talk guinea pig!! We bought ours during the summer too and we've all become REALLY attached to them ... . In fact I can't quite believe how besotted I have become ...
One of our books says that they can withstand the same sort of temperatures as us. Ours live indoors (currently they are in our main living area but they move downstairs to the basement sometimes too) but they go out on the lawn every day to graze if the weather is OK (light rain fine but I bring them in if it's pouring). Right now, I wait until the frost has gone from the lawn and everything's warmed up a bit until I put them out. I bring them in every night.
When they are outside we have them in a sturdy wooden ark which has a covered 'sleeping' compartment attached with a wooden floor - which I cover with newspaper and hay. This allows them to graze, sleep, graze, sleep and we move it around during the day to a new patch of grass. It also protects them from bright sun, rain, or cats which, where we live, lurk around the cage salivating! For that reason, I can't let them out to roam in our garden unless we are there to supervise.
When the grass stops growing and it gets really cold, I think we are going to buy a biggish hutch (on stilts) to put on the terrace so that they can still go outside during the day when the weather is reasonable. I'll put a heavy tarpaulin over the top of this to protect them from the worst of the weather. It's good to ensure that they still get lots of fresh air though.
Personally, I prefer them to have three smallish enclosures (not the smallest by any means but a practical size to clean out) and keep them interested by moving them around ... rather than one big one in which they are bored and nothing ever happens and is a nightmare to clean... I clean their cages every fourth day.
The books say that they have quite poor eyesight and get stressed by an enclosure that is really large. Obviously, they should be able to have a good run around every day though. They always like to have a dark, covered area to retreat to.
If they happen to be in their indoor cage during the day, I take out the little wooden 'houses' that they sleep in at night so they are forced to be more sociable with us - otherwise they just retreat in to the covered area and don't become tame.
I do let them run around the house but only under supervision (we live in an old house which is undergoing renovation). I don't really let them go in to the nooks and crannies as it could be quite dangerous here.
We have more or less "trained" our gps to only poo and pee in their cages. You learn the signs: they either lift their heads up and sort of nod, or squeak when they want to retire to their cages for a "bathroom break"! Apart from when they were really young and peed and pooed everywhere, they are now fairly consistent about only 'going' in their cage .... If by any chance you get pee on your carpet or wooden floor, then white vinegar cleans it up very well.
Our always have a hay rack filled up and fresh water of course and they are fed twice a day. Once in the morning with a bowl of fresh food. I use a round bowl about 7 cms in diameter. They like carrot tops, broccoli or celeriac (especially the hard stalky bits) a carrot, a bit of apple (don't overdo the apple because some gps get sores on the side of their mouths because of acid apple juice), cucumber, and they love chicory .... they also love playing with half a melon. Only give them spinach and parsely in moderation because it contains a toxic chemical. As others have said, take out any fresh fruit they haven't eaten after an hour.
At the moment they graze throughout the day on grass as well of course. In the evening I give them more hay and a bowl of the same size of dried hard food bought from the pet shop (which has added vit C). I also give them small branches of fruit tree wood to knaw on to avoid tooth problems and we add lots of interest to their cages in the form of a tunnel with fleece interior, a willow ball etc etc
We clip their nails with a human nail clipper every couple of months as well. It's a good idea to weigh them regularly so as to pick up any health problems in plenty of time. Being rodents, they hide signs of ill-health until the last minute to make themselves less vulnerable to predators. By the time an illness becomes evident, it's usually too late.
Most of the time, they are petted and played with about twice a day. One of ours is much more docile and she gets more attention. We leave the more neurotic one to have a bit more space ... Sometimes one of them goes to sleep on our lap while we are watching TV .. the other likes to hide inside the lapel of dh's dressing gown (when he is seated).
Our gps have really great, distinct characters and as long as they are handled safely, I think make fantastic pets for children.
Crikey, I've written the GP chronicles ...
Have fun with yours ...