I love cats but I have a soft spot for guinea pigs too.
I think they are great.
You can defo have them in a hutch if you let them out to have a good run once or twice a day - in the garden if it's nice (they do need a barrier or they'll escape, like a rabbit run works fine) or we used to let them on an old piece of rug indoors if it was raining. Sometimes we'd even let them have the run of the living room, but you have to keep an eye on them as they will wee on the floor. However their poos are little hard pellets and can easily be hoovered or swept up. DSis and I used to make them little elaborate mazes out of cardboard crates you get from the supermarket, which was wildly entertaining. Another thing we liked to do was create a "human run" which is where we'd sit with legs in a V shape touching feet to make a diamond/triangle/star (depends on number of people), guineas in the middle and you just use your hand as a barrier if they try to climb over a leg.
They will keep your lawn nice and neat without needing to mow it - just beware of any poisonous plants and don't put the edges too close because they will eat anything, they are nothing if not curious.
You can save your vegetable clippings for them, carrot peel, top of celery, etc. They love these and they get so excited and chirpy when you open the fridge, it's very cute. You may find yourself collecting dandelion leaves and groundsel when you're out on walks because it looks particularly tasty They do need fresh food every day alongside their dry food.
Their little movements are hilarious and very cute. Popcorning as mentioned. When they sit on your lap they will sometimes burrow into your arms/legs and push up with their noses, which I always liked. If you handle them a lot they will be happy to be cuddled. Not quite like a cat or dog, but much nicer than rats (IMO). They can be very chatty and the variety in their noises is delightful. Fur varies - there are different fur styles and some are more fluffy than others. Long haired varieties might need brushing with one of those soft baby hairbrushes. I quite like the ones with wiry, fuzzy hair.
They don't smell at all if you clean their cages once a week. If you want an even easier time to clean out then line their cages with old towels and a top layer of fleece, with hay or straw as bedding for them to burrow in. You can just shake off the poos into the garden and throw the towel/fleece into the wash (no fabric softener) and replace/rotate it.
Do make sure you buy proper guinea food, preferably all one type of pellet, rather than the "museli" type because they are picky and they will pick out the bits that they like which can leave them vitamin deficient.
You will need to clip their nails. If you have light coloured guineas this is much easier to see and you've less chance of accidentally nicking them on a nerve which will upset them. Dark colours are harder to judge. If you feel really nervous about this, a vet can do it for you at their check ups. Or ask the vet to show you the first few times.