Definitely do the tour with Troy of Plymouth. It's incredible. We also went diving with him. Email him before you head down though. I know at one point he was contemplating moving off island due to all the updates to Little Bay. He can also taxi you over to Rendevous Bay (unless you want to hike it 45 mins uphill and back) or rent you kayaks. The restaraunt right behind the dive shop has really good food too.
Spend a day or two or three at Woodlands Beach. You will have it entirely to yourself. Bring snorkel gear and maybe an umbrella and pack a cooler lunch. There are outdoor showers and lots and lots of snorkeling to do a little walk down the beach due to a huge rock field. I saw an octopus on the first day.
Don't plan on spending too much time at the Observatory. It has a nice view and some interesting things but it's quite small and the woman who worked inside was surprisingly pretty rude.
Montserrat is a wonderful place and you'll love it. Just don't expect a Sandals type experience. And enjoy the flight! it's a tight squeeze but it's an experience you'll never forget!