I do not for a second think that the Leafs will sell. But in the name of discussion, here's my answers.
Muzzin and Barrie would be the two best rental defenders on the market. 1st plus a small add for either of them.
Cody Ceci is yours if you want him. Playoffs or not, tbh.
Spezza is probably yours for a 5th.
Kerfoot - Can't see him going anywhere unless we get a 3C coming back. I'm not attached to him by any means, but I don't think he's ultra valuable so there's no real point to selling. 2nd and a halfway decent prospect.
Johnsson - 25 years old on a reasonable contract with some term, scored 20g last year but is having a bad year with production and injuries. The most likely to get moved IMO, I think he's worth two 2nds. Plenty of demand for a decent young depth scorer on a good contract, but not the kind of guy that anyone will break the bank for. For a team that wants someone to plug into their second line on the cheap, he'd be a great pickup.
Kapanen - Far and away the most valuable of the three comparable guys IMO, the other responses in this thread give you an idea of the demand there would be for him. The team acquiring him should beware if they're planning on getting a top6 forward, but anyone expecting him to bring speed and talent to their third line and PK is going to be absolutely thrilled. 23 years old, term on a cheap contract, and 20 goals last year. He's worth a mid first at least, but I think it would take an overpay kind of add to pry him loose.