On the subject of Mill Ends Park (World's Smallest Park), while you're in beautiful Portland, OR, also check out the world's largest city park, Forest Park. (See my "Wild in the City" post--updated today--for more details on this and other Portland parks). The world's largest park of any kind is Greenland National Park at over 80,000 square miles. I've been looking all over the place for the world's tallest structure: a radio mast in North Dakota.
This extremes thing is really cool, by the way. Keep up the good work!