Plugged in will be best, but if you can't do that then leave with it charged to 80-90% then it'll still have charge left when you return.
Turning "always connected" off, will help, although I don't normally bother with that as I have scripts that monitor the car every few minutes and I'm not normally gone long enough for it to matter to me.
Stats from my last trip away (Model S 75D)
Car left @ 85%
2017-04-19 19:40:19, battery_level : 85, battery_range : 216.54
Charge left when I returned
2017-04-28 06:16:59, battery_level : 75, battery_range : 189.97
Without the network access there would be less loss, but even as is, around 3 miles per day is reasonable.