depends what Im doing, how long and where. Standard hill sac for my 'other' activitiy is a Berghaus Extrem Pro. 40 L
For bushy / other stuff i have a Highlander Pro Force 33, a UK Forces Other Arms Bergan and an Outwell 65+10 for multi day stuff. I have one or two other 35/40 L packs kicking about too one Berghaus Airflow, and the other is a Deuter ski mountaineering pack
The only thing I will generally strap to the outside of a rucksack is my Karrimat/skis/snowshoes/ice axe and kit to dry otherwise nothing ever gets attached to the outside.