Just two weeks ago, I finished another demanding travel challenge together with three friends. We have been on a tour around Germany, to log at least 10 caches in each of the 9 neighbour countries within 8 days (GC53CYB Challenge-Wald IX - Rund um Deutschland (8/9/10) http://coord.info/GC53CYB.
I can tell you, we really enjoyed this trip!
Of course, we also visited Liechtenstein, as it was on our way (and necessary for another challenge ).
When we reached Austria, we thought spontaneously about doing 5 countries a day the next day. We had to add a cache in France, as the visit in France was initially planned a day later, but that was all, we had to prepare.
Two days later we started in Saarlouis to finish Luxembourg and Belgium and had the next overnight stay in Maastricht, Netherlands, where we also found two caches after dinner. So, we did the "BeNeLux in just one day"-challenge by chance during our tour.
Here is an excerpt of the caches, we found on 20th of August 2015:
Luxembourg (Letterbox): GC2WDT6 Geo Post Office - Hesper http://www.geocaching.com/seek/log.aspx?LUID=c3478507-bac6-41aa-b265-736c419860da
Belgium (Tradi): GC40 Geocache http://www.geocaching.com/seek/log.aspx?LUID=6be3bc85-d039-4743-bc1c-04ccf4202e76
Netherlands (Wherigo): GC434D3 Mestreechter Staar 10: 'n RUNDSJE VRIETHOF http://www.geocaching.com/seek/log.aspx?LUID=c4d17080-877b-43b5-9f37-4a05a37fb742
When we visited the challenges final on our way back from a meeting in Belgium, I quickly found the cache and we continued our way homewards. TFTC!