I was on your exact situation, 5 1/2 months but without the older kids. In my Lamaze class, they told me to get a wheelchair so I could rest. I laughed at the idea, and DH took no notice. I mean, I did exercise and like to walk. Ok, so I had a little condition that made me be in some pain if I didn't take enough rest, but I thought it wouldn't be a problem. Big mistake. After standing one and a half hour to rent a car in the first day (a hot, hot, May day), things went downhill after that, and on day 3 I had to be in bedrest for a hole day. I was miserable that day, but it worked, and the remaining of the vacations we rented the wheelchair, and it was heaven for me! It worked great.
So, my advice for you: if you are in excellent shape and used to exercise, do as the other post tell you: take it easy and rest as much as you need to.
If not, rent a wheelchair! Your DD2 can even go with you, and you will not have to push it to a extreme like I did.
Enjoy your vacation, and take ALL the precautions that you need to.