If you have 2-3 hours to spend in Montrose, you might consider first visiting the Montrose Air Station Heritage Centre. This museum is dedicated to preserving the history of military aviation in the area. You can expect to spend about an hour exploring the exhibits and learning about the various aircraft on display.
Next, you might take a walk through Montrose Basin, a local nature reserve and bird sanctuary. This is a great spot to enjoy some fresh air and take in some natural beauty. Depending on how much time you have to spend, you could take a short stroll or a longer walk through the reserve.
Finally, no visit to Montrose would be complete without taking a look at the town's historic architecture. You might consider taking a leisurely stroll through the town centre to admire the many historic buildings and landmarks. This shouldn't take more than an hour, but you could easily spend longer if you're interested in taking photos or learning more about Montrose's history.