Ban Nakasang is also the closest large market to Don Kon and Don Det. The town itself is of no real interest to a casual tourist other than to visit the market there and obtain goods that are not available on the islands.
It is unfortunately a rather filthy and uninspiring place.
Use the ATM as there is no ATM on the islands.
There are some places to eat close to the Mekong. Most of the places serve Lao food such as noodle soup.
There are several guest houses in and around the town if you should end up here and wish to spend a night or are forced to.
Buses leave for Pakse from the market. Minibuses leave at 12pm from the two minibus services on the main street and cost 40000. Farther destinations are offered by the agencies, such as Ubon, Thailand and Vientane, which, while more expensive than paying each leg individually, offer more security as they can phone ahead to the bus station to buy the ticket and make sure you get there on time.
Ban Nakasan is close to the Cambodian border (in the south). You can book a bustrip into Cambodia there. Servies are being offered to arrange visa formalities. I did without, and found that it was just 1 USD cheaper. It is possible to reach Banlung in one day, a popular Cambodian tourist destination in the North-East. Destinations like Phnom Penh and Siem Reap take long bus tours.