The only real option you have is to use them. The best way to use them if you don't have enough is to rent additional points from other owners. Use these additional points with your Bonus points to get a room where you'd like to stay.
You could offer them up for rent on here. Just list the expiration date of the PlusPoints. Indicate that they are bonus PlusPoint and that you have to make the reservation for the renter. Of course this only works if you have a fairly large chunk of points. At least enough for at least one nights reservation. Though don't expect to get prime $$ for them given that they are PlusPoints and the restrictions with expiration date and the fact that the renter can't get them transferred to them.Another option if you only have a hundred or so, I did this with 100 left over PlusPoint. Offer them up for free on the Marriott forum on TUG. You could also do the same in the TUG sightings forum if you are a TUG member (the mods may move it there anyway). I offered the points up for free in this thread and I had a taker in just a few hours. Better than letting them expire. I did try to offer them up for free on the old points rental site that GregT had, but I didn't have any takers. That is why I posted it to TUG.If you don't have enough for a single night, then you can work something out with someone else that has points where they tranfer enough to you to complete a reservation and then you make a reservation in their name using the PlusPoints and the points they transfer to you.