My husband and I are looking for a charming hotel or B&B for our last night in Scotland. We would like to be no further than hour from the airport.-preferably about 30 minutes away!
On the southwestern shore of Loch Lomond is the upscale hotel & resort Cameron House Lodges. I believe they had a fire a few months ago, but I'd expect repairs would be done soon, if not already by now. See http://www.cameronlodges.com/
A stone's throw from there and also on the loch shore is Duck Bay, a more mid-priced hotel with a good restaurant that has huge picture windows with a commanding view of the loch.
This is less than an hour from Glasgow airport if there's no traffic and if you don't count any delays with turning in your rental car etc. If you need to fill your petrol tank before returning the car, Dumbarton is the last convenient stop for that. You'd follow the A82 south to the Erskine Bridge, and once over the bridge take the M8 toward Glasgow. But be aware that the airport is actually in Paisley, not Glasgow. The signs for the airport are prominent, but there are a couple of roundabouts once you get off the motorway, and the sign for rental car return is not all that prominent.
About 10 miles north of Duck Bay, also on Loch Lomond, is the very charming village of Luss. (Don't blink or you'll miss the sign on the A82!) For B&B options, see https://www.tripadvisor.com/Hotels-g551943-c2-Luss_Loch_Lomond_and_The_Trossachs_National_Park_Scotland-Hotels.html
Cameron House was pretty much destroyed by the fire. It remains closed, and will do for some time. There is still no published reopening date, and they've laid off most of the hotel staff. Six weeks after the fire, they still couldn't get the investigators in safely.
The lodges, which are a couple of miles further on, are still open along with the leisure facilities there. However, I don't think they meet OP's criteria of 'a charming hotel or B&B'!