Most folks who stay near the ruins eat all their meals at their hotel. If you're looking for variety or staying at the campsite, there are several small restaurants (comedores) between the main camping area/parking lot and the gate at the beginning of the road to Flores. As you arrive at Tikal from Flores, you'll see them on the right side: Comedor Imperio Maya, Comedor La Jungla, and Comedor Tikal are the best of the bunch. All serve hefty plates of fairly tasty food at low prices. You can get a large serving of roast chicken with rice, beans, and fresh tortillas, along with a drink, for around Q40 ($5.35/£2.65).
Within the area of the ruins there are covered picnic tables and itinerant soft-drink peddlers, but no snack stands. If you want to spend all day at the ruins without having to walk back to the parking area for lunch, take sandwiches. Most of the hotels here and in Flores, as well as the comedores, will make you a sack lunch to take into the park.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.