Ssophier...mid September can be beautiful in Glacier, but you can also run into unexpected weather, especially on the Going to the Sun Road, which can cause it to close. Some lodges and services inside and outside the park start to close down after mid-September also. Since you didn't indicate which part of the park you were staying, I'll assume you're spending time on both the east and west side?
First of all, remember you are in bear country, so both you and your husband should carry bear spray, especially since there won't be as many people in the park. Even though you will be on short trails, don't assume you won't see any bears. We've seen bears on the trail when were close enough to see the parking lot.
On the west side of the park, the Trail of the Cedars is a one-mile trail partially on a boardwalk and partially paved through an old hardwood forest and past Avalanche Gorge.
Along the Going to the Sun Road, stop at Logan Pass Visitor's Center and take the hike up to HIdden Lake Overlook. It starts off along a boardway and then there's a fair number of stairs before you get to a regular trail, but it's well worth the effort. You'll have spectacular views and usually a great opportunity to see mountain goats.
A very nice waterfall off the Going to the Sun Road is St. Mary Falls. It's less than 2 miles r/t. If you're up to going farther, Virginia Falls is beyond St. Mary and a spectacular waterfall.
I hope you are planning on visiting the east side, especially the Many Glacier area. It's considered the most scenic area of the park. Take the walk around Swiftcurrent Lake for a great opportunity to see moose and great views of the Many Glacier Hotel.
From the Swiftcurrent Motor Inn in Many Glacier you can go to Redrock Falls. It's 4 mile r/t - but if that's too far, just go as far as Fishercap Lake for another great opportunity to see moose, especially in the early mornings.
For a nice outing, and If Glacier Park Boat Co is still operating, take the boat rides across Swiftcurrent Lake and Lake Josephone and take a picnic lunch to Grinnell Lake.