Feel free to design your garden's perennial/shrub beds and paths without regard to the septic system. Their root systems won't bother the septic system. Keep the septic tank access clear (sometimes it's capped with a lid and sometimes it's buried) so you won't have to destroy plants when the tank needs pumped out. Also, use caution when digging deep planting holes so you don't damage the drainage tile. These days, the drainage tile in the laterals is usually not tile at all, but perforated black plastic tubing. The only two limitations on your garden design are be 1-keep any additional tree plantings off to the sides, as the forest is now, and 2-depending on the system's size and arrangement, it may be extremely tricky to find an open spot for an in-ground pond.