Hi, hope you have a good trip to LOS and welcome to the board.
For your questions
You can apply for a 60 day visa at you local Thai embbasy.. Or you can hope over the boarder, Somewhere like Singapore, Laos, Malaysia etc for a few days and go back in... With Vietnam as you said you could be intrested in, you can book a flight from Bangkok to places in vietnam... I am 99% sure Thai fly to Ho chi minh city and Hanoi..
www.thaiair.com youll be able to look there.
Youll Enjoy elephant riding... Did a day trip from Bangkok to see the bridge on the River Kwai.. Not far away there is a place where you can ride elephants... Ill find out where it is. We finished a bit early on our tour and the tour guide took us there for as it was on the way back to Bangkok.
Go with an empty bag, youll be able to get clothes very cheap all over.. You can get a lot stuff from the night marlets... Patpong in Bangkok is good for that (all fake but who cares) and tailors are plentiful although i have never been to one.. Sometimes inn Hotels (depending on where your staying) They will be able to tell you where a good on is. I know one of the hotels in Bangkok i stayed in did.
A trip to see Muai Thai mainly the Lumpini stadium in Bangkok is a must just for the experience.. The crowds are crazy and tickets arnt that expensive... Although i cant remember the price again of the top of my head.
Im sorry i cant help you with the other questions but it might be worth seeing if your doctor can prescribe you a longer course for time in LOS... Some of the other board members might be able to help you on that one.. Ive no experience in buying from chemists in LOS
Enjoy your trip and let us know how it all goes
All the best
cybernator