Barbara has given you some great ideas and the info on the money exchange.
Velit Bay is also worth visiting and have a good restaurant on the beach there too. It's to the south of Champagne Bay, on the way to the blue holes and Luganville.
I have pictures on most of my reviews of the blue holes, Little Paradise Restaurant at Port Olry, Lonnoc Beach, Velit Bay etc if you are interested in seeing the pics. (I'm not the best photographer though) but will give you an idea. (just go to the Santo ones)
Hope you can get a driver, there is a number of taxi's around the Luganville area, so I hope someone can tell you if they go up to Champagne beach. I think they have to pay the entrance fee the village chief charges to enter that area.
Sure you'll have a great time, we love Santo, so beautiful, peaceful and unspoilt by tourism (so far) and very friendly local ni-vans.