(1) Listen to a Marriott timeshare schpiel. 15K for 2 hours of your time. The Marriott schpiel is what I would call "medium-pressure": they are GOOD at their work (trying to convince you to buy timeshares), but they aren't weaselly used-car salesmen. (No offense intended to actual members of the weasel family.) It's up to you to decide if this is worth the bait offered.

(2) Like Amex, Diner's Club allows Marriott conversions. $1250 spend = 2500 Club Rewards Points = 1500 MR Points. There's a high annual fee, but you'd earn 24000 bonus points in 1 year. A little bit of spending and you'd have 15,000 MR points.

(3) The Hertz option is an excellent one if you happen to have a lot of 1-2 day rentals. For short rentals, the flat 500 MR points per rental is the best option. (Yes, the points actually post!) As you cross into a 3-4 day rental (or any higher-priced specialty rental), Hertz's #1 Club Awards Points become more valuable IF you do enough business with Hertz to reach 600 points (1 Free Day rental).

(4) Push orphaned miles through other programs to get them to MR. I wouldn't recommend burning non-orphaned miles in this manner (unless you find a good bonus promotion), but if you can squeeze a few thousand MR points out of otherwise useless miles, it's better than nothing.

Of course, the obvious method in this case is also the best: stay at a Marriott, pay for your stay with the MR Visa.