In the Colorado Rockies they will tell you there are three seasons, Winter, Summer and Mud Season. Mud Season occurs twice each year and anywhere else would be referred to as Spring and Fall. We have travelled to the Rockies in both Mud Seasons.
The pros of Mud Season are all related to the lack of people. No need for dinner reservations. No crowds to fight off. The paid parking is free. Time share exchanges require minimal points. Hotels and resorts can be booked at cheap rates.
There are, however, still many things to do during Mud Season. There are literally miles and miles of paved bike trails that can afford long or short walks or bike rides. White water rafting can be booked in the fall Mud Season. Jeep and 4×4 tours can be booked. You can self drive a scenic tour.