Is there anywhere else in the area that you would add on to that basic itinerary? (perhaps with two days to play with)
I might have thought that Chandigarh could interest you (if not Mrs Meltwaterfalls!). Overnight train up there from Delhi (saving a hotel too) and you could do it in 1 day return (such a train journey is an "essential" part of the India travel experience anyway). It is around 250 kms from Delhi - the overnight trains go "slowly"! Choose the train which starts in Delhi not the ones which have come up from the south as they can arrive rather late - especially in the winter mists of N India. Alternatively a 3.5 hrs evening train from Delhi with prebooked hotel in Chandigarh.
Another day and you could take in the nearby Kalka - Simla railway. (Another 115kms all the way to Simla). If you only went as far as Kalka, (plus a bit further i guess if you went by taxi - but the whole journey is a slow mountain road taking 3.5 hours) from Chandigarh you could take it in on the same day as Chandigarh and still get back to Delhi that evening. Kalka is less than 30 kms from Chandigarh. You would miss the delights of the journey up to Simla of course and the colonial ambiance of the town. I should mention that, IMO having visited all 3, this is the least interesting of the mountain railways - behind Niligiri, with Darjeeling the best. But still worth seeing. You need to consider your likelihood of taking in one of the others "one day"!