Imagine you are in a stationery shop. You say to the shopkeeper "I would like {this book} and {this pen}" because there are many books and also many pens.
However, if he has a special promotion where a book an pen are sold together (you can imagine the pen is maybe attached to the book) you would say "... this {book and pen}".
"This ... " is simply drawing attention.
To address your second example, you may use either. And each will communicate something subtly different.
If you were to say "this waterfall and fountain", you are grouping them together. As if you can point your finger, and I look to where you are pointing and I see both of them. Or we walk to a particular location, and they are both at this location. It may be natural to make such a grouping. And it may not. It will depend upon the situation. You could probably create a situation where both can be used.
Whichever one you choose, the language has to be correct:
"This {waterfall and fountain} IS ..."
(use singular because there is only one object)
"{{This waterfall} and {this fountain}} ARE ..."
(Now there are two objects, so we have to use the plural!)
Also, "among considerable features" is also not good English. "among THE considerable features" is better. "among many features worthy of consideration" or "among the most beautiful features" are better, although this is a different question.
PS the trickiest situation might be where you have several objects constituting one feature. "these steppingstones {are/constitute} one of the more notable features of...".