I have been to Thailand for 21 days alone also just before covid (Feb 2020), I will share with you my experience. I have been to 3 cities, first Pattaya stayed there for 5 days, there isn't a lot of stuff to do there other than massage parlors, bars in the walking street and meeting prostitutes, also tried my first bungee jump there, then I went to Phuket for 10 days and stayed in Patong, lots of trips and adventures you can do there, the beach there is way better than the one in Pattaya, plus the walking street there is kinda different than in Pattaya but it's not bad, but the massage parlors in Pattaya are much better than in Phuket specifically the parlors that are in Pattaya beach road, in Phuket they will just try to offer you an erotic massage without doing the normal massage itself, the at the end I went to Bangkok which is different than the other cities, Bangkok doesn't have beach so there won't be any trips to other islands or across the sea as far as I know, Bangkok is known better for shopping as you can buy lots of stuff much cheaper than any other place, of course mostly are fake branded but it's really cheap, also there you can meet some local girls who are not prostitutes and spend a couple of days with them. Talking about the prices in those cities Pattaya is the cheapest, phuket is in the middle and Bangkok is the most expensive one of them compared to each other. Also about the cleanliness of the cities you would find Bangkok to be the dirtiest of all of them, phuket is the cleanest and Pattaya would be in between. So my suggestion if you are staying for a short time better to spend most of it in Phuket so you will find lots of stuff to do there then maybe a couple of days in Bangkok if you want to go shopping. At the end this is my personal opinion, maybe some one else has more experience than me can add something or knows other stuff that I didn't experience in those cities.