The ships travel during the day, which then allows the passengers a leisurely view of the passing scenery along the way. At night the ships are docked generally in town. Easy walk into the town areas. Older middle aged folks, depending upon when you visit Europe. Pricing often includes a number of tours. Drinks, gratuities, extra highlight tour(s), laundry, souvenirs are extra charges. The ships generally are small for easier navigation and pass through the locks. Less than 200 onboard--crew & passengers. The Host/Hostess is bilingual, majority of the guests are American. The tour guides are locals who meet up with the ship at different stops. Set mealtimes, some ships are open seating; if a tour group is on board, the dining room may have a section roped off for their group dining. The ships we sailed on had a casual attire dress code with only one smart casual/dressy evening for a meet the captain event. The hardworking crew kept to themselves and spoke little English with the passengers; the Hosts and front desk/reception staff were bilingual.
I would suggest choosing a destination, gather up the brochures, compare their offerings and pricing. Early booking offers some discounts and choice of cabins. Then there are last minute sales. --Another forum for more info: cruisecritic.com