Tom’s Ancient Warfare rules are a set of free wargames rules designed to be played with — of all things — Risk pieces. The author notes that the bases may need to be larger if you use historical miniatures.
Tabletop Titans has a tutorial on using inks on a white base to create a sort of “watercolor” look to your figures. I very much like the look and am trying to decide on what unpainted project I could use the technique. Actually, I’ve been thinking about getting some of those woodland creatures figures and doing a “Mouseguard” game. The watercolor look would be just right for that.
Aqua Zone is a set of free wargames rules for playing miniatures games that feature underwater combat with submarines, DSV’s and marine animals.
Although this video is focused on Warhammer 40K, I think the techniques shown for building terrain entirely out of cardboard are worth studying.