Hotel Krone Unterstrass on Schaffhauserstrasse (
http://www.hotel-krone.ch) isn't directly in the centre of town - it's two tram stops from Central - but has recently renovated its rooms to a very comfortable and modern standard. I've used it when having visitors several times and, as a former travel guide, am impressed with the rooms and especially the staff. Not much in the way of facilities but who needs 5 star? It'll be as romantic as you make it.
Prices around CHF150 inc. breakfast. But go without and eat out for brekkers and save CHF20 or so.
Always negotiate hotel room prices, especially if you can spin some romantic yarn such as yours to the receptionist
But if you really wanna impress, go to the Widder...£££££, it's beautiful.