Only 1 hour from Sukhumvit is Ayutthaya, the old capital of Thailand. There are dozens of ruins of the old city which is fascinating. It is not Angkor Wat but it is still really really cool. Make sure to have a local driver take you around to all of the sites as they are a little spread out. A must do if you visit Thailand.

We spent a whole day travelling around Ayutthaya by scooter looking at the ruins and historical sights.I really enjoyed it as I could imagine how majestic it must have been many,many centuries ago.My teenage children found it a bit boring after a while as some of the places are quite samey.The entry prices are pretty low but hopefully this money goes towards restoration.Some sites had litter which was disappointing but Thailand generally has litter issues.We took some lovely photos at the various sites.I was delighted to get some fabulous photos of the Buddha head in the tree roots,truly an iconic image of Ayutthaya.I liked the fact that you can pick and choose which wats to visit as there are so many .A local map was helpful as was the sign posts around the town.I loved travelling by scooter as it was easy to park and the town wasn't too busy with traffic.

We were actually quite disappointed after we went to Ayyuthaya. It never came near the beauty of the historical park in Sukothai.
Ayuthaya is much more busy and touristy then Sukothai and the ruins aren't that well preserved. It doesn't have a park feeling because the sites are in the middle of the town, so when you are at a ruin, you can always here the traffic. Lots of the buddhas don't have heads anymore, and the ruins aren't well taking care of. At each temple/ruin you have to pay an entrance fee. It's mostly around 100 bath which isn't that expensive.
I wouldn't bother to come here if you have already seen Sukothai. Maybe if you see this before you go to Sukothai, you can enjoy the sites more.