Being a land of rivers and lakes, boat trips are common in Vietnam. Do not miss out on a cruise through Halong Bay, for which you should allot two or three days. You must also take boat tour of the imperial tombs in Hue. Hoi An and Nha Trang typically offer fishing or snorkelling rides. At the Mekong River, going by ferry is more a practical means of transportation than a leisure ride, but it can easily be both.
A fun way to experience Vietnam would be to hire your own car or minibus, with a driver, of course. You can also hire a motorbike or a bicycle, especially if you plan to head up to the mountains. Most of the time, though, rented motorbikes and cycles are not insured. The best way to get around a place, really, is through its local transportation. This means doing as the locals do.
There are numerous options, depending if you are merely getting around a city or hopping from one southern stop to a northern destination. Vietnam has a system of buses that run short or long routes, and most only travel during the daytime. As the drivers are notorious for overcharging foreigners, many cities now have ticket offices with fixed prices. Trains are much safer and more comfortable than buses, but they are really slow, too.
When you are within a city, you can normally get around on an air-conditioned taxi. A more interesting way, though, would be the motorcycle taxi and the cyclo pedicab. The latter can only be found in less busy cities and in small towns. In both cases, though, be sure to agree on a price before you board. Carry exact change, too, so that you can pay the agreed amount if the driver tries to ask for more in the end.