As you can tell by the description under my username, I am a newb. I have a number of questions but will try to do research first so I don't ask something that it is simple to find the answer to. Here's one, though, that I have not been able to figure out through searching.

The idea that I have currently for a novelty/toy has as its subject a US national landmark. Let's say its the geyser Old Faithful. The idea might include an image of the geyser, as well as something like "Welcome to Old Faithful", say. Does the government's Parks Service own the rights to the name of the park? Would I have to apply for some sort of license to use its likeness? What if it does NOT include an image of the geyser, but simply the phrase "Welcome to Old Faithful"? Would there be any difference?

I appreciate any help, even if its just a link to some info I was unable to find on my own. Thanks!

The best way to be sure is to contact the office in charge of those landmark.
For example, vendor required to apply for license when selling goods related to University of Texas (everything, including building).
Meanwhile, manufactures can produce image/printing of Alamo on goods freely...

There is no need to ask permission - this is public domain and nobody can have a trademark with that name.
the only thing you need toworry is not to use someone elses pictures or description of the monument or natural wonder. For ex. I can go to Niagara take some pictures and put them on a T shirt and say "Welcome to Niagara" and sell them - no problem.