The touristy part of Cologne is really quite compact, and a lot of the visitors come via the river cruises. So there are a few quaint streets round the cathedral, then eateries with beer gardens going down to the riverside. Rather than valentine's, I'd try and check out what effect Lent has on general revelry. I'm guessing little effect, and it will be business as usual, but this is a guess. If you were to hit town around 10the Feb, the pre-Lent Carnival will be in full swing and you'll be guaranteed lots of fun and frolics.
Re. the Sunday opening hours, if I were you I'd contact the Tourist Info at Cologne direct, email address at the foot of this page
http://www.cologne-tourism.com/city-experience/shopping.html