Thinning out woodland areas is a great way. Using a heavy chainsaw to fell and buck trees into a size that can be carried is a great way. Carrying logs as well. If it's a large tree that I want to mill I will use an excavator to move it, but only after I clear a path and get the log as close as possible. The major workout for me comes from soil conservation concerns, as I am willing to work considerably harder in order to remain low impact. Moving stones and constructing trails is good as well. Of course this all depends on your biogeography and is site specific.
If you don't have land, it's pretty easy to find land with invasive species or ecologically malignant overgrowth that you would be doing the world a service to restore. I find when dealing with rhizomatous herbaceous to shrubby infestations, like multifloral rose or wintercreeper, a brushaxe or machete often works just as good as a motorized circulating saw type brushwacker. And pulling by the root is often very effective depending on the soil, as it circumvents periodical mechanical clearing and herbicide application, pulling roots can be akin to a powerclean if in good form.
You can also climb trees or even construct an outdoor jungle gym. I live in an extremely steep hilly ecoregion division of the Ozarks so my legs and cardio are well taken care of simply by exploring. Kayaking is good as well.
If you hunt, carrying deer is a good workout, leave the ATV and stand behind, learn how to field dress your game and package it for pedestrian transport. >>1669665
Yes fencing is good. Even better is split log fencing, you can fell trees, split them with handtools and use a joinery mallet and chisel to do the joinery (I use a small chainsaw) stonework for walls and trails is also very good.
Learn basic joinery!! >>1665014
mixing clay with lime and manure, and spreading it with a trovel to seal ponds and pools, or using a lasagna method to produce gley sediments is a good workout