I think Moby Dick would not be appropriate because it spends so much time discussing just whaling boats and keeps missing its point. Melville can be a bit dense .
Travels with Charley IS an easy read and its by a great author who had lots of insites about the country . Again, several levels.
Huck Finn or (less so) Tom Sawyer may need to to have the reader develop a sense of dialect (I always loved reading in what I supposed was the dialect of the characters).
Zane Grey and London are two goodies also and you can drop in some poems by Service to fill in prose and poetry.
Nothings better for prose than to decipher the language that only poetry can provide.
This sounds like a neat project.
Take a run therough THE VOYAGE OF THE BEAGLE. Its more an adventure story.
Are your students mostly rurral or urban?
If theyre urban, perhaps some detective stories by ELmore LEonard would be good reading. They are very easy, Leonard paints his scenes with action (unlike some who just bore the **** out of me with these Bulwar Lytton phrases and run-on POS)