Kasteel Amerongen is a moated, rectilinear castle-mansion whose sumptuous salons can be perused by joining an hourly guided tour (in Dutch). If you just want to look at the rather austere frontage you can stroll down to the very impressive Bentinck cafe-restaurant, housed in converted stables complete with statue horse heads gazing at the tables. In a walled section of grounds there's also a beautiful formal garden.
Originally built in the 13th century, the central building was burned by Louis XIV's troops in the late 1600s, then rebuilt with its present dour appearance by an aristocratic family whose descendants occupied it for the next three centuries. Bus 50 takes 65 minutes from Utrecht (€5.15) and 15 minutes from Doorn (€2.20). Driving or cycling from Wijk bij Duurstede, the 7km route wiggles along a raised dyke with bucolic views, passing a large commercial lock system part way.