Choose mild temperature ranges (perhaps between 60 and 85 degrees F), make sure part of the enclosure is shaded. Provide water.
The lawn should be free of chemicals and not frequented by dogs. Be familiar with what grows in your lawn, as a few things (buttercups, scarlet pimpernel, flowering bulbs etc.) can be poisonous.
Supervision is necessary because no temporary enclosure will truly be secure from dogs. Make sure your cavies cannot crawl under and escape.
Here's a pic of one delux guinea pig run that staindgirl uses: