Train to Tarbert for the Arrochar Alps
Train to Tarbert then walk north over Ben Vorlich (steep) to Ardlui for a return train.
Or do Little Hills and Ben Vorlich from Ardlui
Train to Tyndrum for a big day on Ben Oss and Ben Lui. You could perhaps find a route over the hills back to Crianlarich.
Train to Bridge of Orchy for a steep climb up Ben Dorain
Train to Falls of Cruachan for a walk up Ben Cruachan, return along the ridge to Bonawe station
Train to Taynuilt then NE along the shore of Loch Etive, up and over Glen/Lairig Noe and down to Dalmally station - this looks like a really good low level walk
Plenty of other interesting looking hills immediately south of Crianlarich, but it would appear that access to the hills immediately to the north is hampered by the lack of a bridge.
PS - I'm beginning to feel just a little bit jealous - which is perhaps inappropriate given that I have the Lake District on my doorstep.