Hi Zelena,
Congrats on being newlyweds! Michigan City is an ok stop with a zoo and large outlet mall within walking distance. They have some decent resturaunts including Swing Belly's and a brewery called South Shore Brewing Co.
We have stayed at New Buffalo a couple of times this year and absolutely love it! The municiple dock is in a perfect location between a beach and downtown. There's a "world famous" hamburger place called Redamaks, and our favorite stop, Oinks ice cream is right down the street. A resturaunt/pub called "Stray Dog" seems to be very popular and is in close stumbling distance to the marina.
You can't go wrong with South Haven, but I think the same can also be said for Saugatuck, and Grand Haven! Last year we hopped up the coast to Grand Haven and Saugatuck and absolutely loved both. We still talk about the Filet Minon at "Phils" , a small bar/resturaunt in Saugatuck as being the best steak we ever had!(maybe we were just really hungry)
Good luck on your trip wherever you decide to go, and as always don't forget to post pics of your journey!