Would it be okay to simply omit "one" and say "Well now, how about this" instead?

Is it common to say "joining me today is [someone]"? Isn't is more natural to say "[someone] is joining me today"?

This is just an example of a less common, inverted grammar structure. You can use both "joining me today is" and "is joining me today" to express the same today. This grammar pattern is natural in this situation because the example given is a bit more formal to begin with (introducing someone). Ex: Here with me today is our guest speaker, Dr. Harris. Ex: Our guest speaker, Dr. Harris, is here with us today.

What's the main difference between "hike/hiking" and a "picnic?"

The difference between a "hike" and a "picnic" is that a "hike" requires physical activity. It's the activity of going for long walks in nature or up mountains, whereas a "picnic" is sitting down somewhere, usually in nature or a park on a blanket, and eating food and snacks. Ex: We hiked up the mountain and then had a picnic at the top. Ex: I'm not fit enough to go hiking at the moment. Ex: I love going on picnics! Especially if there's wine.

Is it common to use the verb “plant” to “fire”?

No, it is not very common to use the verb "plant" with the word fire. It is more common to use "start" or "build" instead of "plant." It sounds a little odd when he uses "plant a fire." If you can't decide what verb to use with the word "fire," I would recommend either "start' or "build." It will prevent confusion and you will be sure that you are correct and understood.

What does "hit the books" mean?

This is an informal expression meaning to "study or read seriously, in a determined way." For example, you can use this to describe students who are in the midst of an exam period. Ex: It's exam week, so I saw students hitting the books all over the library. Ex: He was a bad student up until his senior year of high school. Then, he got serious and started hitting the books.

Is "stack" same as "pile?"

Stacking is when you put things on top of each other, so that they are organized vertically. Piling is a similar action, but there is less organization involved. Things might not necessarily be on top of each other. For example, throwing all your shoes together is called piling, whereas stacking them on top of each other in an organized way is stacking. Ex: I stacked all the books on my desk. Ex: I threw all my dirty laundry into a pile on the floor.