Here is a set of 10 "gates" you can use to place in scenery to create a racing, search or navigation challenge
There are in a model lib and texture folder structure so you just include them in your project structure - no seperate libraries needed
I would recommend you put POI's on them - they are quite difficult to see
So create you own circuit, offer them as challanges on this site - ruin the peaceful countryside or annoy students in the Weston Library in Oxford - your choice !
Scale them up for different skill levels
Please credit me if you use them publicly, not to be used in an commercial product