Explore the Rhine. The Danube. The Seine. Lovers of river cruises — and their number is growing, especially among the over-50 set — have been there, done that. Today, what floats seasoned travelers’ boats are the Amazon, Ganges, Volga, Yangtze and more. Book an Irrawaddy River outing, which navigates the waters of the once-out-of-bounds Myanmar (formerly known as Burma). New rivercraft for 2018 include Avalon Waterways’ all-suite 36-guest Saigon, due to launch Mekong River outings in September, and Viking River Cruises’ all-suite 52-passenger Viking Ra, which will ply the Nile starting in March.