Cat D3, Deere 110 TLB, Kubota BX23 and L3800 and RTV900 with restored 1948 Deere M, 1949 Farmall Cub, 1953 Ford Jubliee and 1957 Ford 740 Row Crop, Craftsman Mower, Deere 350C Dozer 50 assorted vehicles from 1905 to 2006
If allowed... the simplest is culvert pipe.
Any type of bridge will need to take into account the load, span and anchorage.
Some have taken old mobile home or trailer frames for small spans...