Draw lots of sketches and see what you can come up with. Make sure to keep the track wide enough (10-30 feet). If you are going to dig a pond for dirt, then put most of your jumps near that area so you don't have to haul as much dirt. Also keep in mind a parking/pit area, room in between the track to get through with equipment (if needed), etc. A track loader and a dozer are the best two pieces of equipment for building... or a dozer and a skidsteer.
Different obstacle ideas: doubles, triples, tabletops, whoops, step-ups, step-downs, rythm sections, big berms, etc.