What to visit (Bangkok)

Hi all, probably I will be in July for business trip in Bkk and I would spend a couple of days to visit something.
Please could you give me some recommendations in terms of:
- what to visit absolutely in BKK (I love photography, then I like nature, temples, markets and everything is authentic);
- where is the best place in the city to stay with the hotel;
- recommendations on restaurants and nightlife are also warmly welcome ;-)

Many thanks in advance!!
Ernesto