I could maybe see Frodo, if he's reading Bilbo's book. But Legolas? There's no place for him that would make sense. Unless he's kicking back at Elrond's or something. It can't be more than a bit part.
Well, his father WAS the king of Mirkwood - and it is known that the Mirkwood elves fought in the Battle of Five Armies, so it would be very easy for him to have a cameo appearance.
They are also bringing back Frodo, and I heard Merry as well (and maybe the others). The way I see it being done, is that it will either be older Bilbo telling the tale to Frodo and the others, or Frodo reading it. It would easily fit too, because the way the Hobbit is written feels like it's being read to you.
The second part of the movie will be part Hobbit stuff (the battle of Five Armies probably, or Smaug ((though I heard they finish with him in the first movie, not sure))) and the dealings with the White Council and their dealing with the Necromancer of Dol Guldur (Sauron), which would tie the Hobbit into The Lord of the Rings.
Obviously there isn't enough source material in the Hobbit to make two full movies, so they are adding stuff Tolkien wrote but wasn't in the Hobbit.