I recommend St. Lucia! It is absolutely beautiful, there are plenty of restaurants in the Castries area or at your high end hotel and the music is a real cultural delight. Once a week they have a "jump up" party in Castries (I think it is Fridays) and it is a street party with live music and food. If you stay in the Soufreire area, you will be closer to the Piton (twin mountains), but more isolated. Ladera resort is a very romantic resort with many of the units open on one side and complete with private plunge pools. Sandals also has several resorts there if you want a large resort.

If you are not worried about pristine beaches, then I second the vote for St Lucia! It is stunningly beautiful and there are plenty of excursions to do. It offers rainforest hikes, some strenous, some not and biking trips. Also has a really fun ATV adventure that takes you to the Atlantic beaches which are more rugged. There are so many things to do there, we had to go back the next year.

If you want to be in an area that is more active, check out the Rodney Bay area. Many restaurants and bars, but not rip roaring night life. Soufriere in the south which I visited only for the day, is reportedly much quieter with less restaurants and entertainment. Some posters prefer this. I like the more "developed" area to the north with day trips to the south, but St Lucia is still pleasantly undeveloped.

Check out stlucia.org and sunlinktours.com for more info about the island. There is also the Friday night jump up in Gros Islet(northern part) and the Friday night fish fry in Anse La Raye, on the west coast. I know someone who hired a taxi for the evening that took them to both.

I also agree that St. Lucia is a beautiful island - we stayed at Anse Chastenet and thought it was a beautiful island with a lot to offer. I have to also strongly recommend Anguilla - it's the only Caribbean island we've been back to many times, mostly because of the amazing restaurants, the friendly/ fun-loving people, and the low-key nature of the island. It really is our favorite - I'd highly recommmend it.

St. Vincent. Just like anything beautiful it is all in the eye of the beholder. Leeward side black or salt and pepper sand beaches, coconut groves, and best of all almost always private. Windward side rocky and black sand, the power of the Atlantic as it hits against the cliffs. Totally awesome. Rain forest,Soufriere, Owia Salt Pond, Falls of Baliene, all places of beauty and grandeur.
Restaurants: Villa across the Channel from Young Island, Young Island, Grandview. You also have the option of day trips to Bequia and Mustique. Islands close enough to visit with little hassle, yet very different from the Mainland. St.Vincent and the Grenadines, my favorite islands.

We imagine that is what we are going to find in Nevis this winter. It has not been overdeveloped. The beaches are supposed to be more of a darker brown sand than many other islands, but I am told it is absolutely relaxing. No need to make restaurant reservations from home but yet gourmet meals. It sounds customized to meet your specific tastes when you say hiking and natural beauty. We chose it for many of the same things you are looking for.