If you visit Plymouth, you can see THE Plymouth Rock. While smaller than most people think, it is a part of history, and I think everyone should visit it at least once. It's free to see, right on the waterfront, and about a 2 minute walk from the Mayflower II.

Everyone knows Plymouth Rock, but how many have visited Plymouth's other historic rock? Sacrifice Rock on Old Sandwich Road is the Antiquarian Society's oldest and perhaps least known historic site. Centuries before the arrival of English settlers to the area, this ancient landmark was an important stop for wayfarers. Travelers left small branches or stones atop the rock perhaps as a gesture of sacrifice, or to receive the blessing of safe passage -- the full meaning of the custom is shrouded by time. (from their website)
Located on Old Sandwich Road, where Sacrifice Rock Road connects, it's about the same size as Plymouth Rock, but you can actually touch this one.

Uluru is one of Australia's most recognizable natural icons. The sandstone formation stands 348 m high and measures 9.4 km in circumference. The Aṉangu Traditional landowners lead walking tours to inform visitors about the local flora and fauna, bush foods and the Aboriginal dream time stories of the area.