We visited Lord Howe in late October 2012 - the daily temps were about 22 degrees. Could not resist a couple of quick dips in the lagoon water as it looked so nice - but it was a bit chilly.
Not too bad after you get used to it though.
There is a track that runs parallel to the road where you can walk along and enjoy the lagoon views. Our flight also came in to land across the lagoon so we also got to enjoy the breathtaking views from the air.
There are various lagoon front sheds where you can hire paddle boards etc.
The lagoon is just like a picture post card - too good to miss.