If you're in a business setting, don't discuss business over dinner. In some situations, business can be discussed over lunch, but let your Finnish host bring up the topic to avoid offending them.
When eating at a sit down restaurant with a server, you should round up or tip about 10% of the bill. In bars a tip is appreciated but not necessary.
Drinks
Milk is served with many meals in Finland and is, in many ways, the de facto drink of the Finns. For many however, they prefer their coffee, perhaps only to wake them up. No matter your preference, all popular drinks are available, including soft drinks, juices, tea, and others.
For alcoholic drinks, Finland is well known for their vodkas, some of which have been popular international exports. On most evenings out for a Finn, though, beer is the drink of choice, although nearly everything is available, including other beers, hard liquors, and wines.
Generally speaking, the tap water is safe to drink in Finland, 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.