I doubt you'll fill up all your cards while in the parks, and if you have your laptop with you, you've got your portable hard drive right there with you back at the room. As long as you have enough cards to last you through a day of shooting, I wouldn't bother with a portable hard drive. I'd likely leave my shots on the cards AND download them to my laptop each night. If for any reason I ran out of space, I could then erase a card and start anew, since I've got them all copied to my harddrive...but I'd feel even better if I had all the pics on both the card AND the laptop!
I'd just make sure to bring a USB cable for the camera, or a card reader, so you can drop everything on the laptop as a backup, and keep it all on the cards unless and until you run out of room.