Sorry, I wish I could help, but it seems the only suggestions your getting, (even from the Okw lovers), is what they can do away from the resort. There really isn't much to do AT the resort for nineteen year olds. I guess your' best suggestion for them is to walk over to Downtown Disney, do Disneyquest on and off during the day, and if you're comfortable with them going into pleasure island, do that.
Drop them off at a monorail resort, and let them pool hop between, the GF, Poly, and Contemp. They're sure to enjoy the younger crowds there. They can rent one of the small boats there if they want. They are a lot faster than the Okw and BW sprites, and they can actually go somewhere. Our 19 year old and his friends liked going to the contemp. to (show off) play volleyball and meet other kids their age. They had a great time at the Coronado Springs. They met some people their age, and later set up times to meet up with them throughout WDW. If you wanted to drop them off at Universal, they have a lot of things that cater to teenagers at citiwalk. Only problem is picking them up after a long day for the parents. If your' son at 17 felt there was nothing to do for him at Okw back then, he will definitely feel the same way at 19. Check out the calendar of events for House of Blues(have a lot of great concerts), Downtown Disney, Hard Rock at universal (they have nights for the 17 and older crowd, without the drinks), and whatever maybe going on at that time. If he has a chance to plan something, before he goes, it may be more enjoyable. Good luck!