Is it a sin for married couple to stay alone outside in nature, and to have a intimate contact, but to make sure no one from people sees them.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Yes, it is permissible for a married couple to have sexual intercourse in a place where they are sure that no one can see them. There is no doubt that people go to the places that you mentioned in the question; however, if the married partners are sure that nobody could see them, then we do not see any harm in that [there is no prohibition in it].

Allaah Knows best.