Kailua is a wonderful little town on the Windward side of Oahu. The park itself is beautiful, large and has barbeques, bathrooms, lifegaurds, etc. Locals tend to cluster there on Sundays but are very friendly to "mainlanders".  Most weekends local bands play while kids and adults hang in the water. This is not a beach that must be given over to islanders on the weekend. Everyone is great. You will find the "Little Green Store" very close and can get sandwiches, salads, beers...everything for a good picnic if you haven't gotten it together for a whole grill-fest.  

Lanakai is very calm and beautiful. Kailua is pretty windy but excellent for boogey-boarding, kayaking, and swimming. Look out for the Portugese Man of War after the ninth to twelfth days after the full moon. You can see them on the beach and that should be a good warning.  Walk before you swim. Lots of good restaurants are located in Kailua. Try Buzz's and Lucy's. Here, there are lots of soft white coral sand beaches and a fantastic island experience. Once there, you might not want to leave.