As a parent, if you have a chance to attend, I highly recommend it. As you can see from the schedule, that Marciemi posted, there are several meetings/panels scheduled for parents that are very informative. The tour of the yard as well as the tour of the STEM building was very impressive.
My son brought along a sleeping bag. For clothing he wore khakis and polo shirts. You don't need to bring suits or ties, but remember you will be attending classes with your mids and I think it shows respect to the mids and the professors to look decent. If you are going to wear jeans make sure they don't have holes! He brought work out clothes (he never used his, but like Marciemi pointed out, other candidates did).
I do suggest bringing something for the midshipmen who you are staying with (cookies, candy, snacks, etc). My son brought homemade chocolate chip cookies. The mids he stayed with were very appreciative.