Let's start with your statement about renting an exchange. That's against I.I. rules. If they catch you (and they watch these threads), your membership can be suspended or revoked and, worst of all, your exchanges cancelled leaving you to deal with some very unhappy renters.
You can call interval and cancel your exchanges. You should get replacement weeks for future use.
Now, cruise vs land vacation. I have a different point of view than most on these forums. We are avid timeshare vacationers and avid cruisers. Both have advantages.
If you really don't think you'll ever return to the islands, I'd choose a cruise. Cruising is more of a sampler platter or tasting menu. As mentioned you won't get the full flavor of any one island but you'll get the experience a little of all of them. It's a great way to experience a little of everything.
We have enjoyed our timeshare vacations in Hawaii because we can spend as much time and see as many things (or as few) as we please and do them on our schedule. At this point, I prefer the timeshare vacation in Hawaii rather than cruising but, that's because we've been here and know what we want. If we'd never been to the islands and if a great deal on a cruise came up, we'd have taken the cruise for our first experience of Hawaii.
Of course there's a lot to be said for the cruise experience, the fine dining every night and service you recieve onboard. Having meals taken care of in the price of the cruise is a bonus. Grocery prices on the islands and/or eating out can be cost prohibitive. For instance, a quart of milk cost us $2.49, a pound of frozen shrimp was $26, a 12 pack of Siera Mist was $7.99 (but they had it on sale for $5.99), breakfast cereal was running $6.75 unless you bought what was on sale, A box of ginger snaps was $5.49 and a small tub of Blue Bonnet margarine was $2.49 (I'm looking at my reciept as I'm typing).
Don't forget the cost of a rental car. We got a good deal on a compact for $197 for the week plus one day (taxes and fee's included) but it's not always that way. I've seen complaints of the cost recently as high as $475 for a week. Gas isn't cheap either at $3.89/gallon and, then there are all those pesky speed traps I've seen on our trips. The only ticket I have in the last 8 years was on Kauai where I got caught in a section where the speed went for 35 mph to 25 mph for no apparent reason.
If you like cruising, it's a great deal and you really believe this will be your one and only trip to the islands, I'd go with the cruise. But I'll warn you, in 2009 we thought that would be our one and only Hawaiian vaction. I'm typing this from our unit at Hilton Hawaiian Village on Oahu and this has been our third trip. We're already contemplating when we'll return and which island we'll return too.
