A few people have mentioned Geocaching to me in the past, but this weekend we were inspired by my in-laws to actually give it a go. Geocaching at its most basic, is a huge, outdoor treasure hunting game using GPS-enabled devices e.g. your smartphone. Sign up to the site for free, and you'll have a map of all the locations of the registered caches along with further details on each one. Participants are given a set of GPS coordinates and then attempt to find the geocache (container) hidden at that location. Depending on the type of cache, you may find small items there to swap, or just a logbook to record your visit, but you have a guide as to which it will be in advance, based on the size of the cache marked on the map. There are literally thousands of these caches hidden across the world, and it is finding them which is the real challenge. 
For us, it was a great way to get the children excited about going for a short walk to find a cache spot, armed with a small, unopened bottle of long-lasting bubble mixture to swap! As GPS is never completely accurate to the exact spot, you'll arrive in the vicinity, but still have the challenge of actually finding the hidden 'treasure'. They really are hidden too, if you think that you've probably unwittingly walked past lots of them before, whilst going about your normal business!
Get the children even more motivated by pinpointing caches found on a map on the wall, or cross them off a print-out from the Geocaching website. This is a free activity and is great for getting children out on walks! You can sign up to get premium features for £22 per year if you want to take it more seriously and unlock some additional features. Equally if you're keen, you could set up and log your own Geocache for others to find; lots of possibilities :-)