The Model Permission for Encroachment is a license agreement that allows an encroachment to continue until the owner withdraws permission. The model document also features owner protections from claims pertaining to injury or property damage, a requirement that the encroacher comply with the law, and the termination of any rights the encroacher may have to claim adverse possession.
Southwest Michigan Land Conservancy uses this spreadsheet to calculate stewardship costs for a fee-owned property with trails, a parking lot, signs, and other amenities.