I also live in a Cape Cod and I absolutely love it! At least you have a great start in your flower beds and you can always add more to spruce up your house and yard. If you want to add flower boxes, I would only put them on the first floor windows. I think it would look weird on the other windows.

As to paint colors, there are so many out there that would be beautiful with white. Think about colors you and your wife love and I would like to suggest you choose a separate trim and door color to spice things up. Your shutters could either be painted the trim or door color, whichever you decide.