You could take a look at this site
http://sites.google.com/site/wandelcaches/Home
wandelcaches = walkingcaches
at the left you can choose a provicie (state), next you get the caches with walkingdistance specified. also you read "kaart met wandelcaches in Friesland (for example) when you click on this it opens in google maps.
Hope this helps you.