On the GRiSO, (& others with low bars & rear sets) a small backpack make a lot of sense. Go with the Kriega or similar (motorcycle specific, with chest & waist straps, very comfortable). For touring add in the water bag. Keeps you hydrated. Keep the pack small & the weight light.
I regularly take along a Sea To Summit silnylon daypack - the waterproof one with the roll top I find best - folds up small, usually lives in the tankbag until shopping for the evenings groceries. Also convenient to keep food clipped to the ridgeline overnight to keep it away from the possums & other fauna sharing the campsite. At times on the road, I use it for a few light bits & pieces I want conveniently accessible - the merino 2nd layer, a spare pair of gloves, the uber light down jacket I use as a liner - that there's no room for in the other luggage. Oh yes, it also holds a six pack comfortably - if that goes on the rear rack, you risk losing the tops where the road into the campsite is less than smooth.
