Okay, any treat will work here, I just happen to love Jelly Belly Jelly Beans, and they are perfectly kid sized.
You will notice when travelling with young children that at some point they will just be "done". Whether you get the "I'm bored" or "I'm tired" or "I don't want to walk anymore", you'll know they've reached their peak. Sometimes, this will NOT conincide with when you are done.
One way we keep our kids going, is with motivational Jelly Beans. I usually carry a baggie of them in my day pack. When I start getting the complaints, I'll use the Jelly Beans to tempt them on. I generally only give them one at a time, and only after the kids have completed a request ("Be quiet for five more minutes and you can have a jelly bean" or "Just walk to that stop light down there, and you can have a jelly bean"). I know bribery is "bad", but it has worked out for us. A lot of times it takes just a few beans,and they forget that they are bored/tired, etc.
Find something that your children like or want, and use it as a gentle motivator during your explorations. I would recommend combining your "motivational jelly beans" with praise...."You are doing such a good job" or "You've walked a long way today and I'm proud of you", it seems to make the treat even sweeter!