Depends on how big a trailer I'm taking.

and whether I'm out for the weekend or just an overnighter. then I don't need the sleeping bag

. I never know what I'll need. Usually:
eyepieces from 32mm down to 6 mm and celestron filter pack (used twice) and 2X barlow
Red touch and The Southern Star Guide and The Cambridge Star Atlas
Laptop and 12V adapter/240V powr cord
12V power pack and Ciggie lighter double adapter or if power is available 12V regulated transformer
Fresh AA (torch) and C batteries (RA Drive on Celestron)
300D and ToUcam
80-200mm zoom and teleconverter
Card Reader.
6.3 Focal Reducer, Tele-extender, "T" adapter and "T" adapter mount, OAG (never used, still take it anyway just in case

)
Hartmann mask
4 point safety pack and extension lead
12V Blower Heater, (Laptop doesn't do a good enough job removing dew)
Meade wedge and fittings and bubble level
Weights.
Screwdrivers and allen keys
Esky and 6 pack of rum and cola (golds usually)
It starts to add up after a while. I can usually cut this down by a third to a half if I plan what I'm after first, and whether I'm viewing or imaging