Mystery Spot 11 is the Logan Pass Visitor Center in Glacier National Park. Situated on Going-to-the-Sun Road, where it straddles the Continental Divide at 6,646 feet, the Logan Pass Visitor Center is one of the National Park System's landmark structures.
To start you on your way to a solution, you were given this limerick:
Two drops land side by side,
But because there is a divide,
Downhill they start,
Ever further apart,
Until the end of their ride
The limerick refers to a Continental Divide, though it isn't clear which one.
To lock it in, you were given these additional clues:
• The original Gap store was opened in San Francisco nearly 900 miles from here.
• Bound for a solar encounter, you can stop and ask for directions here.
• The Maple Leaf and Old Glory face the sun together.
• Old McDonald had a farm, Mary had a little lamb, and Boston has an airport.
(1) The original Gap store was opened in San Francisco nearly 900 miles from here.
This clue indicates that the hydrological divide referred to is the Great Divide (aka Continental Divide of the Americas), which separates the watersheds draining into the Pacific Ocean from those draining into the Atlantic Ocean (including those draining there via the Gulf of Mexico as well as Hudson Bay). Use of the word Gap -- a synonym for Pass -- provides an additional hint.
(2) Bound for a solar encounter, you can stop and ask for directions here.
Glacier National Park’s renowned Going-to-the-Sun Road crosses the continental divide at Logan Pass. (Precipitation falling on the west side of Logan Pass flows to the Pacific Ocean, while that falling on the east side flows to the Atlantic Ocean via the Gulf of Mexico.) Traveling Glacier National Park's Going-to-the-Sun Road will have you -- at least metaphorically speaking -- "bound for a solar encounter." If you need directions or trip planning tips, a visitor center is a good place to get them.
(3) The Maple Leaf and Old Glory face the sun together.
Glacier National Park is a component of Waterton-Glacier International Peace Park, the world's first peace park (proclaimed 1932). In keeping with the spirit of the international peace park concept, the flags of both the United States and Canada fly outside the Logan Pass Visitor Center.
(4) Old McDonald had a farm, Mary had a little lamb, and Boston has an airport.
Boston Logan International Airport, which was named in honor of local war hero Edward Lawrence Logan (1875-1939), is called Logan for short. The lines from the nursery rhymes could be of some help too. The Sun Road follows the south shore of Lake McDonald for quite a ways, and the most photographed scene in Glacier National Park is the view of St. Mary Lake and Wild Goose Island as seen from the Sun Road while looking toward the Continental Divide.