Ayuthaya is the site of restored ruins from Thailand's previous capitol. It is similar to the ruins at Sukhothai, but not as old, and with a different architectural style. It is heavily visited by tourists, since it is less than 2 hours from Bangkok. The ruins are a little spread out, so it pays to hire a driver for the day (about 800 baht, maybe...).
There's an elephant "stable" where tourists can feed elephants, or even take a ride. Riding elephants is one of those things that after you do it once, you check it off in your book-of-life, and then you never want to do it again.
There's a charge (of course) for visiting the temples, but each site has it's own fee, as opposed to charging one big amount to see the entire park. If you are on a tour, this will usually be taken care of for you.
The temples have been restored by the Fine Arts Department, and so you will often see new materials supporting the original stone blocks. Sometimes this is in the form of concrete, or new laterite blocks, but it generally does not detract from the overall atmosphere of the park.