I love Hawaii so I'm jealous as hell.

Your biggest issue travelling with a 4 month old will be Air Travel. Kids of that age don't understand the concept of releasing the pressure from their ears. It's the main reason kids cry on a plane. Make sure they have and use a pacifier or feeding bottle and have it in their mouth on take offs and landings.

Do the Hannah road day trip up to the 7 sacred pools.

Go up to Haleakala to see the Sunrise. You need to be up at 3am to get there in time & bring really warm cloths. I wore jeans, tea-shirt & flannel shirt and damn near froze & we are from Alberta.

Snorkeling is great down by Wailea in front of those two big hotels, can't remember the names. South of there is a nude beach if you're into that, or at least there was years ago.

Best snorkeling is out at Molikini Island not sure of spelling but there is lots of advertising about it.

Watch your time in the tropic sun. You can easily ruin a holiday if you aren't careful.

Have fun!