I always enjoyed orienteering - the mixed challenges of running over rugged terrain together with high speed mapreading and running a compass bearing.
And ever since I was a kid I've been fascinated by 'time capsules' and buried treasure. I remember hiding a Horlicks jar in the garden once, filling it with a couple of old penny pieces and a tea-stained bit of paper masquerading as parchment with a treasure map of Ipswich.
Letterboxing on Dartmoor is a pursuit that goes back at least a century, as far as I know - it's a sort of mixture of the two. You go off hiking among the peat bogs and granite tors, and aim to find one of the 'letterboxes', which has a stamp inside it - you stamp your card, or exercise book, to say you passed this way. I've no idea why there aren't letterboxes in the Lake District or on the Yorkshire moors... but there aren't. AFAIK.
Now I've just discovered geocaching, which is basically letterboxing plus technology. Youlook up a cache you want to find on the geocaching site, then you follow your own handheld GPS to find the cache - a hidden container with various goodies in it. The rules are; take anything out, it has to be replaced with another object of similar value. There are 'event caches' - parties or gigs you have to find via GPS; there are multi-stage caches, basically a sort of treasure hunt, where each cache contains a clue to the next. There are 'earth caches' - sites of geological significance.
While geocaching has taken off fastest in the US, there are geocaches all over... I'm going to enjoy looking for some in France. And come to think of it, what could be more fun on a holiday to another country than taking one day to do a bit of geocaching? It would give you an additional interest, show you some places you probably wouldn't see otherwise, and maybe introduce you to local geocachers. What fun!