The quickest way is usually iDine. Sign up at http://diningformiles.idine.com, spend $500 at iDine restaurants that earn 10 miles/$, and you've got your 5000 miles. Sign-up is instant and it only takes a couple days for the miles to post after you spend your money.

DL is indeed a NW partner now, but you can't transfer miles back and forth. If you can 25k DL miles, though, you might be able to get a NW award ticket with your DL miles.

Or you could just buy the ticket. Trying to amass miles in a hurry (in the hope that a standard award will be available for the day you want on short notice) is a risky proposition.