River/canal cruises sound like a great idea - the waterways of the UK are often overlooked, but can be incredibly relaxing, beautiful places. I'm a particular fan of canals (not least because of the pleasure of cycling along the towpaths).

Most waterway holidays are self-driven, though (sleeping on the boats), so it depends if that's something you'd enjoy. The classic destination for it is the Norfolk Broads (Wroxham and Oulton Broad are the big start-off points), but there are plenty other rivers canals with good possibilities (e.g. the Great Ouse at Ely, the Wey at Guildford, the Kennet and Avon at Aldermaston or Bath or the Oxford Canal at Heyford). All let you explore a scenic bit of the country at a very leisurely pace.