Somewhere in America is always a good bet as availability is better than other typical 'holiday' destinations.

I had the same 'dilemma' - I have just over 100,000 miles and a 2-4-1 voucher. I've always wanted to visit Cape Cod and it's one of those places on my 'to visit' list that I never seriously considered simply because I've been to America already. It so happens that a friend and I have to be in BOS later this year, so I have combined the BOS trip with the Cape Cod one and we will both fly J for the cost of 2 Y tickets, which makes the holiday very affordable and attractive, as without the miles and voucher we'd have to pay £2,000 per ticket (no chance) or travel in Y

I posted about this and someone suggested CPT for the distance, but it came down to where I really wanted to go this year. Like others have said, it's best to go somewhere you would have gone to anyway, and in my case I would have made this trip with or without the voucher/miles, but I'm more excited about it than I would have been had I just booked 2 seats in Y.