For any trips I would want the 5200. Maybe start with that and decide if you can't strap it down enough for day hikes. I run the 3000 Ultra Bag for a day bag. Lighter material and more streamlined than the Icon Pro. IMO no need to go smaller than that for a daypack as it straps down to nothing - the 1800 might be a lil snug you stuff it with an extra shirt, top and bottom raingear, kill kit, snacks, spotter, tripod, shells, lunch, cap, gloves, gamebag, water, and such.
ON sale you can get the ultra bag for like $65