Here are some suggestions for the missing words in the text:

1. **them** - "popular pastimes"
2. **increase** - "build"
3. **keep** - "stay"
4. **while** - "and"
5. **why** - "why don't you"

So the completed text would look like this:

"There are a lot of pastimes that young people like, and one of (1) **them** is mountain climbing. It is an active leisure activity that people can do either indoors or outdoors. If you go climbing outdoors, it is a great way to (2) **build** your strength and enjoy nature at the same time. You can even set up your own climbing wall in your backyard or inside your home. This way you don’t have to go out and still can (3) **stay** fit and enjoy the activity.

So how can you start mountain climbing? Many people take a class to learn how to climb and use their equipment (4) **while** others join climbing clubs to make new friends with the same interest and go climbing together. If you are an active person and like outdoor activities, (5) **why don't you** try this activity?"