We did the Jacobite from Fort William to Mallaig and then the ordinary train back again. We tried to get into first class on the Jacobite but it was fully booked. You are a bit squashed in standard class as the seats are a bit smaller than normal and by the time we travelled it was fully booked.
It was fun though. It attracts enthusiasts and groups so there was a good atmosphere, everyone friendly and excited. People were really nice about making sure that first timers got their photographs of the viaduct and the train slows down as it goes over to give everyone time to take photos and enjoy the view.
I suppose it depends how enthusiastic you are about travelling on a heritage steam train. My H loves that sort of thing and I thought it was fun. The views are just as good on the normal train though, if that's the draw for you.
There's a choice of ferries from Mallaig to Skye and the small isles. Mallaig is worth staying a night in: you can watch the boats bring the catch in and eat fresh fish and seafood in the restaurants.
From Fort William, we got the bus to the Ben Nevis visitor centre, took the cable car up to the higher level where there was a cafe and easy walks, suitable for us at our age! Fabulous views. There's a tourist information centre in Fort William who gave us the bus timetable and told us where to catch the bus.
Don't forget your Avon Skin So Soft, to keep the midges at bay.