We also have a little one coming in December.
As far as changing diapers, Any flat space will work (so long as it isn't moving). I remember one time changing my son's diaper on the arm rest of the front seat of my pickup. You are going to get so much practice at changing diapers (double with twins) that you will be able to keep one hand on the baby the whole time.
Just remember to have the new diaper ready to go before you take the old one off... Just in case the kid decides to act like a fountain.
As far as what kids need, it is love and attention and stimulation. At first, just having a variety of colors will stimulate their minds. As they get mobile let them check out objects. (So long as it wont hurt them) let them feel different textures. For the first three years of our son's life, we lived in a 35 foot trailer. And traveled all over the north western states. When we were stopped I would try to find something new for him to look at or touch or play with. Now he is at the head
of his class and doing work ahead of his grade level. Kids don't need stuff. There is a whole world out side of your boat that they can experience when you go ashore. And even better yet, they can experience it with you. Encourage them to explore.