It is oft repeated that you need to keep the "outing" in "Scouting". Have a minimum of an overnight trip each month. Ideally, make it a rain-or-shine overnight, but if your troop isn't at that point yet, at least have rain-day plans (indoor bounce house, swimming pool, museum, etc). Keep those plans in reserve and pull them up as needed. Maybe even to the point of having swimming belt loop and merit badge requirements to dangle in front of them on such a trip.
In months with predictably poor weather, do a cabin trip near a ski / sledding area, or an overnight to a Youth Hostel in an adjacent city with cool attractions, etc.
My preference would be something like, "Second full weekend of each month, we do a trip." and politely explain to anyone who wants you to reschedule each month because of their vacation / school break / second-cousin's wedding that consistence is worth a lot.
If you have the volunteers, you can expand to first Saturday is a day hike, second weekend is a overnight, third Sunday afternoon is a service activity followed by pizza, etc. Not every parent has to go on every trip. If a few parents can handle the day hikes, other more experienced leaders can focus on the overnights.
It is also very positive to have a big, iconic trip out there in the future. A week-long, summer backpacking trip, for instance. Don't be afraid to set limits - it is better if you do.
"You must complete 4 of: cooking, camping, first aid, emergency preparedness, pioneering, or swimming merit badges."
"You must have attended at least 8 of the previous overnights."
"you must have advanced a rank in the last year."
You want the big, iconic trip as a carrot for more active participation by the more experienced, more self-sufficient Scouts. Doing so will help you keep the older boys for longer. If a week-long backpacking trip seems like too much this year, aim for one next year and spend a week at a BSA base camp or a national park and do a bunch of day hikes in the area.
edited to add: lay out a calendar at the beginning of each year. As an attraction. For planning purposes. So the Scouts and families know this troop plans to be active.