This article is an excellent reminder of the potential dangers of the lawn to one's pet.
It emphasizes the importance of watching out for chemicals and pesticides used in lawn-care that could be hazardous to a cat or dog. Naturally rodenticides aimed at mice or rats also fall under the warning category, as do any traps that might be set for moles or gophers.
Besides the type of chemicals or pesticides used on a lawn, take care to avoid having actual plants or mulch that would be toxic to a pet if ingested. Azaleas, calla lilies, dumb canes and oleanders are plants which could potentially harm if eaten.
As always, consult a veterinarian immediately if you believe your pet has eaten anything potentially toxic.