There is an entrance fee of 57 pesos to enter the park. This is a large site, a lot of walking is required as there few other ways to navigate the complex, unless you have a car, then you can freely drive around the perimeter if you are staying at the hotel in the park or heading to one of the many restaurants. There are tractor-drawn wagons with seats and shelter that run on a schedule known only to them. If you go by bus, they will deliver you to one spot, from which you will be required to walk to and from. If you tire easily, pack light for this excursion i.e., no backpacks, heavy purses, etc.
There are plenty of friendly park police there, they may limit your driving around the site unless you are staying at the hotel. Taxi drivers are not allowed to drive you around the site, you must have a destination, like a restaurant, inside of the park. If you are adventurous and lucky, you may be able to rent a bicycle to ride around the perimeter on the cobble stone road a bit bouncy. If you are visiting the site and do not have an opportunity to explore the perimeter where the shops, restaurants and old buildings are, you are missing out. Just a little creativity should help you find some transportation inside of the complex. The locals are very friendly and a few pesos will go a long way. Try to at least find a ride around the perimeter to view the complex. It will be worth the effort.