All F2P quests are listed here : http://ddowiki.com/page/Category:Free_to_Play_quests

In the Harbour, not far away from the Harbourmaster, there's the level 12 (?) quest called "Invaders !", which seems to contain Beholders, too.

And then there are - of course - the Cannith Enclave Challenges, too.

Mired In Kobolds is also level 13, it can be found in The Twelve.
It's end fight is quite difficult, imho. (Don't want to spoil it. )

There are a few more quests to be found in The Twelve ("Delirium", level 15, extremely difficult, imho, and "Acit Wit", also level 15).

Edit : I just found out that "In The Demon's Den" is F2P as well, and it can be found in the Inspired Quarter (entrance close to Kobold Assault quest giver).