I write this post in order to recommend you places that you should visit if you have only a day or 24 hours in Bangkok. Anyway, I would like to say that Bangkok has a lot to see and one day would definitely not be enough. Anyway, if you decide to be in Bangkok only one day, this is the recommended itineraries from me.
First Itinerary (Temple Itinerary)
There are tons of temples in Bangkok. For me, visiting temple is one of things do for this city and I suggest you to start the day quite early because Bangkok is hot all year long.
There are 3 temples that are in the list of Bangkok attractions and you can visit them all in a day: Wat Phra Kaew (and the grand palace), Wat Pho, and Wat Arun.
Well, you should bear in mind about the dress code. In order to save your time, I suggest you to dress properly. Some temples have clothes available for rent, but you might face a very lone line.
In the evening, I recommend you to go to Asiatique where you can chill out with the river views. There are many shops and restaurants around, and I am sure you will have a great time there.
Second Itinerary (Shopping Itinerary)
As you may know, Bangkok is a paradise for shoppers. There are many markets and malls around the town. If you decide to spend a day in Bangkok with shopping, I recommend you to read the link below and choose the malls or markets that you want to shop. Believe me, only 2 places you would get tired with shopping.
Well, if you happen to visit Bangkok on weekend, shopping at Chatuchak, the biggest market in Thailand, could be your choice.
In order for you to see Bangkok as much as you can in a day, the places that I recommend you could be reached by boat, BTS skytrain, and MRT subway. All those places in the first itinerary (Temple Itinerary) are located along the Chao Phraya River, while almost all shopping places in the second itinerary (Shopping Itinerary) are located along the BTS Skytrain. Hope you enjoy travel Bangkok.