Get out of Port Vila is my advice or stay at your Island resort. 15+yrs ago Vila was slow and quite and nice, now its a bit of a hot mess.
Its been a couple years since Ive been there, been there 4 times, but the Mele Cascades waterfalls is a nice trip. Take your cossies, towel and reef walkers and a packed lunch so when you get to the top, you can swim and you can swim in some of the swimming holes on the way down. You can do it as a tour but we just took a bus/taxi and arranged with the guy to come back and get us at a set time - saved lots of money.
Hideaway Island is nice. Its a approx. 10-15min bus ride from Vila and a short walk down the road but a boat will take you across the lagoon. They have a beach restaurant, scuba shop, pool and a few snorkling pontoons. They use to have a underwater post box you can mail special post cards from, think its still there.
If you want to go scuba diving, Maggie and Mike at The Big Blue.
There are a few operators that will do a day cruise with snorkelling opportunities, some have canoe trips as well. Oh and by "bus", I mean their mini vans with a B on the front. Be sure to check with your resort/hotel the current bus fares as some will try to overcharge you as youre a tourist.