Bring some toys that they are familiar with, self play is great no matter where you are. If you bring something they already know how to use you wont have to teach them how to play with it. They will already feel confident with how the toy works.
I highly recommend prepping for this and play with the items you think you might take beforehand. Camping can get pretty dirty. You don't want to take some things that wont hold up to a good washing after. You'll see that there is some overlap with thing you can take to the beach or to the sand area of a playground. Maybe even in your own back yard.
Here are some items that could be a lot of fun.