< Are there are other ways to drop into towns such as Gruyeres and Montreux on our from Berne? >
I would say yes, although there is a tradeoff and you have to make a decision. The issue is your luggage. Timetables for end of december are apparently not available yet, but if you did the trip next sunday it would look like this:
1. Arrive at Bern station around 08:00 am, and send your luggage to Lausanne (How to Travel, top of the screen), except your hand luggage, of course.
2. Board train at 8:34 am (note, as there is a train every hour, you can decide to leave at 9:34, 10:34, etc.) , change at Fribourg, then Bulle, arrive at Gruyères at 9:59 am.
3. If you stay in Gruyères during 4 hours, including lunch, you board the train again at 1:59 pm, change at Montbovon and arrive in Montreux at 15:13 (that’s 3:13 pm, but you better get used to the 24-hoiur system used in Europe).
4. From Montreux there are plenty of trains to Lausanne until late in the evening. Pick up your luggage at Lausanne station.
http://www.myswissalps.com/ti metable
You could also visit Fribourg. If you skip Gruyères, you could take all your luggage with you and store it at Montreux (and Fribourg) station(s).
Sure, once your luggage is in your rented car, you get rid of the luggage problem, it’s a trade off as I said. Between Montbovon and Montreux, you would be on the Panoramic Express line. The road uses an entirely different itinerary.
it should be exciting no matter what you decide!