Magic Tree House Collection: Books 9-16 by Mary Pope Osborne
In this series of books, the kids use the treehouse to visit many past periods, primarily to gather books for Morgan Le Fay. In the process, they earn their master librarian cards. The trips include some visits to historical periods and places that are well known as well as those that are obscure. You would think that they would know a little bit more about the time periods they are exploring. There are also bits of "ghosts" that save them from some challenges. (I could do without those.)
9: Dolphins at Daybreak
They visit dolphins and communicate with them and other sea animals. (They also need to run from sharks)
10: Ghost Town at Sundown
They visit the wild west and meet a guy that is trying to protect his heard from horse thieves. He has a knack for writing and the kids encourage him to pursue the path. They later discover that a book they read about the period was written by him (and dedicated to them.)
11: Lions at Lunchtime
They get to hang out with animals in Africa. Lions end up under their tree.
12: Polar Bears Past Bedtime
They play with polar bear cubs. Um, that does not seem like something that would be very smart. The natives of the region mention they learned a lot from the animals, but I don't think it was that way.
13:Vacation Under the Volcano
They visit Pompeii. They like the Roman times, but wonder why the soothsayer keeps saying the end is near. Then the realize the volcano is about to erupt and barely make it out in time. (But not before rescuing some books.
14: Day of the Dragon-King
In ancient China, the dragon king wants to destroy books (well actually "proto-books", they have not yet invented paper.) They rescue some.
15: Viking Ships at Sunrise
In Ireland, they encounter vikings and end up sailing away in one of their ships
16: Hour of the Olympics
The kids go to the Olympics and discover that only men are allowed. The girl tries to do some trickery to get in, but ends up in trouble.