Other than Green and Jack Johnson (Who I don't envision being traded anyway due to the playoff race) a lot of the UFA rentals are 2nd-pair tweeners at best. But for a playoff run, those type of guys can definitely elevate their own game anyway.
I would assume Green would cost a 2nd + decent prospect.
If the Penguins find a trade for a 3C with another team, they need to shed some salary. I'd personally like to hang onto Ian Cole, but I assume the Penguins want to trade him for a 2nd to make it worth their while. Cole could be a good fit with the Ducks, as his game definitely raises to another level come April and on. For that reason, I'd prefer the Penguins not trade him and let him ride out his contract. But it's not without the realm of possibility. Ian Cole has had a lot of success with Schultz the past couple of years, easy to envision the same level of success with a guy like Fowler. Cole can also play both sides. Usually LD when paired with Schultz but has played quite a bit of RD as well.
Other options for the Ducks perhaps, Barberio has had a good year with the Avalanche, so if they stumble I assume he could be had. Just another tweener is all. The price wouldn't be outrageous at least.
Perhaps De Haan could be had from the Isles, but again, playoff implications. If they do wish to trade him, I'd imagine it'd be for bottom-6 help, and by the sounds of it, you don't want to trade any of your forwards, etc.