I've just moved from Hawaii, and got engaged on the Big Island. This of course makes me the foremost authority. (well, maybe not)
Anyway, Hilo is infamous in Hawaii for lousy hotel options. However, there is an inexpensive, not-too-shabby option: Dolphin Bay Hotel. I've stayed in 4 or 5 Hilo places, and this is the one i'd recommend.
Personally, i wouldn't spend a ton of time driving from one side of the Big Island to the other. I'd just base myself in Hilo. Kona has a lot of sun and some great snorkeling, but it doesn't have an especially beautiful or tropical feel to it, IMO. The #1 thing to do in Hilo, and i'm not kidding, really really...the farmers' market on Wednesday and Saturday. GREAT fruit and other stuff. I'm planning in March when i go back to Oahu to take a day in Hilo just to hit the market mainly. The Tsunami Museum is interesting also. The waterfalls north of town, especially Akaka Falls, are worth a little time.
The Volcano is best viewed at night. Find out where it's flowing, and plan to walk out as close as legally allowed (or closer) in the late afternoon, equipped with a couple flashlights for coming back. Don't worry, there will be 50 or so others with the same plan, so you won't be alone. I haven't done the Pu'u O'o hike yet. It sounds really great though.
If you're using the "blue books" as your guide book, they're great. But keep in mind that you need to go online and get all the updates. For instance, the Kilauea Crater rim is now closed. Entirely. It started exploding and collapsing last year, so a whole bunch of stuff described in the book is no longer accessible.
In Hilo, my favorite restaurants are Pescatore and Ken's Pancake House. The former is a decent real Italian restaurant. The latter is basically a truck stop type of place, but a real landmark, and 24 hours. Your best bet though is the market.
On Maui, geez never mind, i realize i'm typing a novel here. Email me or ask here with more specific questions. I have lots of opinions on Maui too. We're planning the wedding for there. If you're doing Haleakala and have the time/energy for a 1 or 2 day epic, climb it via Kaupo Gap. It's spectacular, but LONG. 20 miles, 10,000 feet. If you're more reasonable/normal, drive up. Lots of nice dayhiking in the crater, and just below it in Kula also.
The road to Hana is great, but get the blue book or some other good guide. Take your time. Explore randomly. Maybe avoid Blue Pool, since there's been some serious run-ins with the locals there recently. Waianapanapa State Park is my favorite place, so i'd personally drive the Hana hwy. in the afternoon, camp at the park (permits in advance) and then come back the next day.
La Perouse for snorkeling is great. Don't waste time on the Molokini boat tours. Blah. Pacific'o is the best restaurant in Hawaii that i've tried. It's in Lahaina. But i keep hearing raves about someplace upcountry that Oprah goes to. Have to try it sometime.
Be glad to show you around if i'm back there when you are. Aloha.