Time is running out for your character(s) to interact with the strange portals on the free side of Runescape. This is a quote from the http://runescape.wikia.com webpage entitled "A strange portal" at http://runescape.wikia.com/wiki/A_strange_portal: "They will be present in the game throughout the whole of 2016."
There are two things worth obtaining (in my opinion) as a result of finding (a sufficient number of) the strange portals. Number 1 would be Skilling speech bubbles. I mean, how cool is that that your character conveys to those around her that she is thinking about Mining as she labours over a patch of Runite ore rocks?
Number 2 would be unlocking the 15th Anniversary Dance emote, which is incontestably the longest emote in game.
Enjoy.