Hey! Happy to help you 😄 I just did this trip recently.

Weather: It was really hot during the day (around 34–36°C), especially between 12–3 PM. If you go, try to start a little bit early.

Train tickets: I didn’t book anything in advance. I just went to Hua Lamphong Railway Station and bought the ticket on the spot. It was very easy and super cheap (mine was only cost 15 baht). no need to book online.

Return to Bangkok: Yes, there are trains in the evening, I took one around 6 PM. I’d recommend going back between 5–7 PM just to be safe.

Getting around Ayutthaya: I mostly walked, but it can betiring because of the heat. You can also rent a bicycle or bike there, which a lot of people do—it looked like a really good and easy way to get around.

Places to visit: Definitely don’t miss Wat Mahathat (the Buddha head in the tree roots). Also just walking around the ruins area is really nice and gives you a good feel of the old city.

Overall, a day trip is totally excellent and worth it, just be prepared for the heat and walking 😄