For families, the beach at the Hilton end of Waikiki is the best. The surf is fairly gentle and lots of people around. Mostly sandy bottom.
The Arizona Memorial is a must for first timers. Beware: yes, you can take the city bus for .50 each way, but it takes forever (so many stops). There are .95 specials for shuttles and they are well worth it. You can cut at least three hours from this visit if you take a shuttle from your hotel.
Quick Tips:
Bring as much food from the mainland as you can. No fresh fruit allowed, but canned and plastic goods, as well as cereal, bread, peanut butter, jam, crackers, etc. is worth it. Bring a case of bottled water -- rooms have a nice refrigerator (we were in the Ali'I tower, so check if your tower has them).
Best Way To Get Around:
Public transportation to Arizona/Missouri/Hanauma Bay is available, but shuttles may be better way to go to save time (it's not that much more money).
When you arrive in the HNL airport, there are kiosks all around with island magazines. Get a couple of copies of "Oahu Gold". Very good info and discount coupons. Be sure to look for taxi coupons in the back of the book. You can get a real cheap fare from hotel to HNL for 1-5 persons (no luggage charges) for around total.