If you're at a business dinner, don't discuss business unless your host approaches the topic first. Meals are meant to be a time to socialize and get to know each other, not to weigh the conversation down with discussions about money or a looming deal. In these business situations, the inviter is expected to pick up the bill, however in more information situations, the bill is divided evenly.
Many sit down restaurants include a service charge, but if not, when eating out you should tip about 10% of the bill.
Drinks
Slovenia has nearly every beverage available including juices, teas, coffees, and milk. There are also soft drinks, including all popular international brands.
Slovenia has found itself in between a number of great alcohol-producing countries so has had little need to produce their own drinks. High quality beers from Austria and wines from Italy are popular, however the locals have still found a few great locations to produce their own wine, but not at the same quantity as a country like Italy. International wines, beers, and hard liquors are also widely available.
Generally speaking, the tap water is safe to drink in Slovenia, but check with locals for any particular regional differences. Also, many people may have troubles adjusting to the local tap water, as it will most certainly be different from what your system is used to.