St. Peter's square in Vatican City is paved with thousands of square, black stones, but there's one stone that's painted red to mark the spot where John Paul II got shot. It's pretty tough to find without looking it up.
Heating water to its triple point (which is a pressure and temperature at which something will melt, freeze, and evaporate at the same time) and watching it freak out as it tries to change each phase at the same time.
Triple point video: https://youtu.be/r3zP9Rj7lnc
We put 27 in the backyard. The kids only found 24, so there are three eggs somewhere.
Next year I'm making a map
[roadside attractions](http://www.roadsideamerica.com/) Check some of these off in your area or on a trip.
In my state those painted rocks that people hide in public places are pretty popular. I have found 4 so far. Some of them are really painted well.
Check out atlasobscura.com There's something neat in your city or nearby that I guarantee you didn't know about even if you've grown up there.
Geocaches are real life Easter-eggs. Fun to hunt down, rewarding to find, and entertaining to participate in.
Houston used to have a random button in the park.
No idea if it's still available.
Carnival cruise ships have a view deck out front that hardly anybody knows about. It's not hard to get to, but it's pretty darn impossible if you don't know exactly how because the exits to it are well concealed. It's great to be out there when arriving at or leaving a port, since everywhere else is crowded. I've had it all to myself before. :)
That's a cathedral built in Washington DC, built only using medieval methods, but apparently out of all the gargoyles, one is H R gigiers alien, and another is Darth Vader.
If you wake up on easter around 7am, most kids will still be asleep and you can search backyards for the easter eggs their parents hid the night before.
When you get a "where's george" dollar bill and you look online to see it's journey to your wallet.
There's this really cool statue of a golden sphere cracked and broken open with another ball inside it that I saw when I visited Trinity College in Dublin. Years later I had to do a double take when I saw the same exact sculpture at the Vatican Museum in Rome. I looked it up and apparently the same guy constructed that sculpture in different notable locations all over the world. I always thought finding more of them by accident would be cool. The sculpture is called "Sphere within a sphere" if you want to look it up
In the city of Minneapolis. There are mini "elf" or "mouse" doors hidden around the city. People have met quite a lot of effort to try and find them all, and new ones are still being found.
Birds! This is the whole reason bird watching is a hobby. They fly around, you see, and sometimes a really rare or exotic one might end up in your neighborhood. You won't know it unless you study some basic taxonomic ornithology.