What is this existence ? What are we doing here ? How did this all started ? How is this different from a dream ?

The mystery of existence seems to be a good idea to cultivate lucid awareness, specially because the mystery is really bottomless and you can never claim to have fully grasped, unlike a conventional RC in which you get a definitive answer. Here, you can not claim yourself to be fully lucid, and so you can not apply to stop practicing.

The meditation upon these topics could well be turned into a practical RC habit, i guess.