If you're travelling from Kyle to Inverness, you'd probably be better going right through. The first train leaves Kyle at 06:22, the next one is at 12.05. Assuming you don't want to get up so early as to catch the first, you'd arrive in Inverness at 14:40. Your only possible return train leaves Inverness at 17:54. At least there's something to do in Inverness.
If you get off at Achnasheen, however, you'd arrive at 13:14 and the train back leaves Inverness earlier and comes in at 14:51. There's not a great deal to do in Achnasheen, although it is lovely and quiet.
Or there's Lochluichart - train arrives 13:31 and departs on the return trip at 14:34.
Theese are weekday times, I've not looked at Saturday or Sunday.
The whole line's really interesting. I'd make a day of it and go right through.
Whatever you decide, enjoy it.