Hi Stamp - are you enjoying your guinea-pig sitting? They are lovely little squawky little creatures aren't they?
At the moment our boys go out when someone is in the house. We have some evil predatory foxes living on the garage roofs so I wouldn't go out and leave them.
We've got a rabbit run (metal bars) about 84" long and knee high (it's tall enough for a rabbit to stand upright buy GP don't bother will all that activity) But
it's not got a base- GPs don't dig but rabbits might. We use metal tent pegs to anchor it but it does need moved every 3-4 days. I can put boxes and tunnels in and I've got an old oldcloth table cover which I peg down to shade it.
I'm at work 3 days so they go out in the evening on those days (6-8pm) then a cuddle then bed. They've got a fan in their shed to stop them cooking 
I don't let mine free range indoors (too many places to hide or wires too chew) but their Pighouse is a playhouse/shed so they've got space and boxes. In winter they are in at night but go out with the radiator and a huge haybox stuffed with hay (They are spoiled, you might have guessed) .
I bought a paddling pool for indoor playtime.
The runs/hutches attatched- they are more difficult to move.
You need to make sure it's foxproof.
Weeds like buttercups and daisies. If you have a seperate run, you can avoid them .
And our run folds flat for storage, or emergency accomodation in the garage (as we had to do last year. Had a leaky window so the GPs had to 'camp' in the garage for a week)
My GP2 (who died) used to get soft pooh after too much grass. My GP3 is a piglet so he's building up his grass time.
They're out about 6 hours at the moment but they just lard about in their houses.
They are lovely little creatures
. We had our Playhouse (wooden Swiss Chalet style) that the DC outgrew so DH adapted it for GP living. The run was about £50 and the indoor cage £40.
Go for the most room you can give them. Maybe C&C cages if you keep them inside?
Last winter I put the rabbit run in the garage (lots of cardboard on the floor) and they had a guinea village to run about in.