Well, these are in my humble opinion of course but these are 2 of my fave islands so here goes...
Maui
1) Hana Highway as a whole 2) Blue Pool (on Hana Highway--buy Maui Revealed by Wizard Publications) 3) Waianapanapa State Park--black sand beach (Hana Highway) 4) Oheo Gulch-aka Seven Sacred Pools (Hana Highway) 5) Any of the beaches near Kahului/Paia to watch the kiteboarders and windsurfers. (Grab a shave ice at Aloha Island Shave Ice by the Maui Crafts Guild--YUM!) 6) A day shopping in Lahaina 7) Snorkeling at La Perouse' Bay/Fish Bowl 8) Kaanapali Beach for a lounging day 9) Iao Valley/Needle (kinda touristy but still pretty) 10) OLD LAHAINA LUAU. An best luau in ALL of Hawaii! (If you're going during whale watching season (Dec-April)--put that on the list)
If you can afford to, a day trip to Lanai is really awesome. Expeditions takes you for $52 round trip (http://www.go-lanai.com/index2.htm) and you can rent a Jeep for $175/day. I know it sounds insane but since Dollar is the only offical rental car agency on the island then they can charge what they want! ;-) Only 30 miles of paved road on Lanai so the rest is off roading and it's one of the most fun things ever. We take the 6:45am ferry to and the 6:45pm ferry back. Might want to think about it.
Kauai
1) Spouting Horn (Poipu) 2) Waimea Canyon/Kalalau Lookout (may have to park at Kalalau Valley lookout and hike up to Pu'u o Kila Lookout b/c that will give you the view of Kalalau you've seen in pics as the road closed to vehicles right now) 3) Polihale Beach (far west side just before Na Pali Coast) 4) Hanalei Valley lookout (Princeville) 5) Hanalei/Hanalei Pier 6) Kilauea Lighthouse and Bird Sanctuary 7) Wailua Falls (Wailua) 8) Opaeka'a Falls/Poliahu Heiau/Coco Palms (before they tear it down) (Wailua) 9) Ke'e Beach/beginning of Kalalau Trail for nice views (Haena) 10) Kauai Products Fair (Kapaa)
I would HIGHLY recommend the Maui Revealed and Ultimate Kauai Guidebook (buy 'em both at Amazon.com for great prices and delivered straight to your door). They are the best guidebooks out there with locals and visitors alike!
Have fun and feel free to email me if you wish! Aloha! Angie