At most places, leave the coins. If you choose to leave a tip, I give it directly to the person or people, and do not leave it on the table. Most restaurants include a service charge, but some do not. If service is included, it should appear on the bill. If service was outstanding and you want to leave something extra(which is normal) give the tip directly to the person. That is how I do it.
I ate at Cabbages and Condoms in Bangkok before I read the first page of the menu that said gratuity was not included in the bill, leave what you felt appropriate. I felt pretty stupid because service was always great and I assumed, wrongly, that service was included. The moral being read the bill or ask if service is included.
At bars a small gratuity to the bartender is nice. If you had a couple of drinks, 20-40 Baht is probably enough, if you stayed all night, a little bit more.