We don't have a guinea pig (sadly) anymore, but we do take our house-bunny away in the van with us. If space permits, he travels in his smaller 'van' cage (as opposed to his large dog crate that is home normally) in the car but if space doesn't, then a useful cat carrier comes in handy with the main cage travelling in the van ready for his arrival. He travels very well and is more than happy to stay in his cage in the van which just stands on the kitchen bench at night if the other bed is in use. Having the split door on the van with the bottom half fastened, he is more than happy hopping around the floor, but his favourite place is actually sitting on the chest at the front of the van looking out of the window and yes, we have had some strange looks, especially when he moves and passers by realise it's not a stuffed toy!
No different to taking a dog away, just a different pet, but still part of the family!