It has been an awful summer in terms of getting piggies out for lawn mowing duties.
Now is the time really that piggies should not be on grass, we have already had some ground frosts (south-east at least) and the ground is cold, as well as very wet. Guinea pigs being so low to the ground are at high risk of picking up infections, particularly with their private parts being low to ground also.
Ideally if at all possible, it is better for piggies to be indoors over winter, but with the right setup, they can be kept comfortable outside in hutches, hutch huggers, bottle huggers, microwavable heat pads, and lots of hay.
If you do decide to bring them indoors, you can definitely keep them entertained with bordem breakers, such as toilet rolls stuffed with hay, paper bags stuff with hay, and if you are able, floor time where you can hide vegetables, lots of chubes, tunnels etc to keep them amused.