I had him look at the Satmap Active 10. It seems like a very simple system (my dad doesn't want lots of options and buttons), and it has a topographical map. There are two drawbacks: the unit costs quite a mint (EUR 400), and the map only covers half of the Netherlands (and an additional map costs another EUR 100). Oh, and that price excludes a bicycle-holder for the unit.
And, even more important: the maps don't offer turn-by-turn instructions for cyclists. Yes, you can scroll over the map and put waypoints at the intersections, but you have to do that yourself. You can't enter an adress or coordinate and have the machine tell you how to get there.
Still, it seemed like the best game in town, because there aren't any auto-routing maps for cyclists anywhere to be found.
Until I got wind of this today. Waypoint, a retailer specialised in (Garmin) GPS units, has created an autorouting map for bicycles -- and it has a built-in bias for 'nice' paths and roads. Exactly what the recreational cyclist wants!
I may need to visit that store with my dad some time (they opened a third retail location close to where my parents live), and have him informed of the possibilities. A GPSMAP 60Cx with this map would cost less than EUR 320 -- quite a difference, and with more capabilities!