Assuming that you stay in Bangkok for 21 days post-op, you really won't have much time or energy for tourist activities during that period. i am a Chett girl and you arent't even allowed out of your hotel grounds until your big post-op check-up at his offices, which is normally around 14+ days post-op (give or take, my memory might be playing tricks on me).
I am pretty fit, heal well and had no complications/required no revisions. Once I had my check-up, I really had no desire or energy to walk around the city or do tourist things. The most I did was a couple of trips to the shopping center across the road and then a walk back from the Vertical Suites to my hotel, the Dusit - which I guess is about 1km.
My advice: be kind to yourself and don't put any stress on your body during your recovery period. Do your sightseeing before your surgery or come back to Thailand in the future and when you are fully healthy i hope to make it back to Thailand one day to be a non-medical tourist :-)
Good luck with your surgery!
