Biatholon: Combine some target practice with your run. You pick the distance. Add occasional stops for some sort of safe target practice: throwing rocks at a stick as it floats down the river or bring a tennis ball and try hitting a particular tree or sign -- use your imagination! My favorite on a wooded trail run with family is to bring a few throwing axes and stop at dead trees (it will draw some attention). Not only does the target time give you a fun break, but it also adds some hand eye coordination. And kinda sneaks in some interval training. Include some friends and have a blast!
Climb On: Kids figured this one out a long time ago: climbing stuff is fun! Next time you are out for a walk or a run, find something to climb along the way (remember to be safe). I like climbing trees - they are nature's jungle gym. You don't have to climb high to have fun and to work some different muscles, just get those feet off the ground!