depends if I'm in for a full day or a banzai run.

BANZAI RUN:

8 mm end wrench
Volvo OEM 10/11 and 12/13 end wrenches (surprisingly good, and light)
Leatherman - or similar
Double-header Volvo OEM (or similar) screwdriver

If you're after trim and interior bits, this is all you'll need. Well, add a Torx set if you're after late-model bits. Nice thing is you can get a lot of good parts with this, and it'll all fit in your back pocket.

FULL DAY:

As above, plus:

3/8 ratchet, long handle, plus 8-19mm sockets, deep (I have a really long handle Matco ratchet. It pretty much precludes the need for 1/2" sockets)
Socket extensions, 2*3" and a 6"
Diagonal cutters (often more useful than the cutters in the Leatherman)
Metric internal hex (Allen) set
8, 10, 13, 17, 19mm end wrenches
#3 Phillips screwdriver
Water pump pliers (Channel Lock)
"Tommy bar" from OE tool kit - good compromise between weight and strength)
8" wide jaw adjustable wrench
Can of PB Blaster
[MOST IMPORTANT] Tool bag with shoulder strap

The "full day" kit weighs about 12 lbs. I find that unless I have a very particular part I'm after which requires some special tool, I can get 95%+ of all the parts off a 240 with this kit. The shoulder strap tool bag is a real back saver. This kit pretty much stays in my car at all times, just in case the mood strikes me.