I also used to live on Oahu - also military (and young and of course not much money).
Now we own at Marriott's Ko Olina, and go every other year for two weeks. We always use airline miles (otherwise the flights for 4 of us would be a lot!), and these trips (minus the timeshare purchase of course) are our most economical vacations!
I would suggest looking into renting at the Marriott Ko Olina - we've used redweek.com for rentals at other Marriotts before. You'll pay a lot less than if you booked directly through Marriott - but you'll be treated like an owner (no maid service, except for one midweek tidy, but we don't need maid service daily in a condo). We like it over Aulani (yes, I know we love Disney, but the Marriott condos are bigger, they have gas grills which we use all the time to cook dinner and I don't think Aulani has them, plus if you rent from an owner you won't pay for parking, as Aulani owners still have to pay for parking, plus the resort is more spread out; we just like it better).
We eat in most of the time, with occasional lunches out while sightseeing. We shop Costco & Target for groceries, but there is a new super Walmart right up the road too, so next trip that's where we'll shop first!
I've been twice to the PCC, and would honestly skip that. If you do want to go, go EARLY so you can do all the little shows. To me, it's just too touristy.
If you drive over to hang out and see Waikiki, you can park at the military hotel hourly (Hale Koa) even if not military, and it's the best place to park (and not expensive) to walk up and down Waikiki.
There are a ton of fun things to do on Oahu - drive over to Pearl Harbor - go early!! I also agree to go to the Flea Market (it's also called the swap meet) - we go there at least twice each trip. You can get tee shirts and other souvenirs, plus dried fruits, nuts, etc... Also, go early - it gets hot!
We like Ko Olina, because the manmade coves are so nice to swim, it's a great area, and Waikiki is just so crazy busy - expensive dining options abound there. We'd rather go sightsee a bit, or hang out at the pool and beach, then grill some nice fresh fish or whatever and eat in at night.
You definitely don't have to spend a lot in Hawaii - just plan accordingly. If it's dinner time, and you're in Waikiki, and you have no plans, you can end up spending a lot on a mediocre dinner. Also most Waikiki hotels charge for parking, and we like to have a car (at Ko Olina you really need one).