Hello,
This post is my contribution to the group effort to learn land nav better. I own a bunch of Ranger Rick books and in some of them he talks about building land nav courses for Soldiers to train on. I wrote an app for Garmin watches for DIY land nav, and I hope it's useful just like some of those articles in those old Ranger Rick books.
The app is "datafield" for the newer generation Garmin GPS watches, which means it runs whenever you run an activity. It is free (no in-app purchases or ads) and the source code is also posted online so many of you might be able to make it better. Unfortunately it isn't super easy to use for someone looking to immediately go out and use it, but it should work okay once you set it up and understand how it works.
Pros:
Once you install it, you plug in the boundary of the park/nature area you're at, and it'll give you a random point to go to (or tell you distance and direction). Once you arrive at that random point the watch will beep and you press lap, and it will present you with another random point to go to. If you encounter impassable terrain you can press a key combination to have the app provide you with a different point to go to.
There are some other options, and some technical know-how is required in order to install it and "upload" the boundary data to the watch, but it shouldn't be too difficult.
Cons:
It can only take a limited number of boundary points due to the size of the app (fewer points is better, ideally less than 8, over 10 and you're pushing it).
Another cautionary note is that many military printed maps out there use old datum (like from 1927), which means the MGRS data on your map may differ from the GPS on google maps or your watch (the correct data). This means that when the watch gives you an MGRS to go to, you may mis-plot it on your map. three solutions to this are: (1) get newer maps, (2) figure out the X and Y offset and apply those differences to the MGRS coord given by the app and then plot it and locate it with your map, or (3) set the app so it shows self-correcting distance and direction, and skip the map or plot it using distance and direction instead of MGRS.
Links
This app should be useful for practicing dead reckoning and practicing pace count! Finally, check out "Orienteering" it's good cardio and a lot of fun. It's also a good way to improve at terrain association and picking your route!
link to app in garmin store - Also some basic information about what this app does here.
link to instructions in garmin forums (instructions as of May 2017 also attached to this post as pdf file)
link to sourcecode in github