This probably sounds like a lot of hard work, but it was some of the best fun I’ve had on a bike in a long time.
Ayutthaya city is as flat as you’ll get and really easy to navigate. Try to plan a route of all of the different sights and activities you want to see and do. You can download a tourist map of the city here. I went to most of the sights on the outskirts first and cycled around to visit most of the others in the middle of the city last. I did this in roughly four and a half hours. I did not go to all the sights and I spent between ten and 20 minutes at the different attractions. I also had a fairly brief lunch, so it might take you a little bit longer. The nice thing though is that you’ll be on your own time and at your pace.
I will be honest though that it can get/feel a little a bit dangerous when you have to cross some of the busier streets, but it will be easy to do if you are patient and tactful. Please be careful though.
I took a minivan to Ayutthaya City from Bangkok and managed to find a place to rent a bike really close to where the van dropped us off.
The minivan dropped us near the train station on a random corner where tuk-tuks with “guides” were ready to pick up and show you around, but there were also a couple of shops with scooters and bikes for rent.
All of them asked the same price of 40 baht for a bicycle or 250 baht for a scooter for the day. I really wanted a mountain bike so I shopped around a bit until I found a shop up the road which had fairly new mountain bikes with aluminium frames for only 80 baht a day.
I would recommend getting a mountain bike, as you get to change the gears and paddle a little bit easier, should you want to. The downside is that you won’t have a basket, like all the other bikes, to put your stuff in, so pack a backpack.
Here is a tourist map of Ayutthaya City: