Best Caribbean island for romantic vacation ?

I've been all over the Caribbean and I personally don't think Aruba is the place for you. It tends to be an older crowd who own timeshares, it's more American-y, it's small boring island, and it's expensive compared to other islands.

I really think you'd like Cancun or down the beach at Playa del Carmen. It's not technically an island like you wanted as it's Mexico but it is awesome. So many all inclusives to cater to you, great safe night life for younger couples, activities like parasailing, you can take a day tour to Mayan Ruins Chitzen Itza and it's really affordable. My boyfriend and I are going in April for $1200 each all inclusive, flying from Boston. (I've been before, he hasn't)

If you really want an actual island, Jamaica is great. The all inclusives there are more high class than Mexican resorts and include golf, snorkeling, pretty much everything. Don't go to Ochos Rios, go to the Negril or Montego Bay side. I haven't actually been but my brother went on his honeymoon and loved it and my parents go there every fall.

Or have you thought about a cruise? It's not quite as relaxing since there are a lot of kids at the pool and no one will serve you lobster on the pool deck lol, and drinks are NOT included so that might be a deal breaker for you. But if you guys have never been to the Caribbean, it's pretty cool to be able to see 5 islands instead of just one. And there are plenty of snorkeling and all that at each island. Southern Caribbean on Celebrity is my favorite...boyfriend and I are taking this cruise in November! (Again, I've been, he hasn't)
There are a lot of great places out there. St. Thomas and St. Marten are amazing but the resorts and airfare there cost double what they do at Mexico and Jamaica that's the only reason I didn't suggest them. But hitting them up on a cruise is a great way to see them.

Barbados at The Crane. I went there in December and it was wonderful and quiet. Excellent views and people were so friendly because I kept getting lost trying to find my room. I am very prone to that.

Most of the Caribbean islands are nice. It's all about the resort that you choose. What is your budget like? Spend a little time searching on tripadvisor.com or go talk to a travel agent. Apple Vacations representatives are very helpful and can help you choose and island and a resort.
I would suggest that you take a cruise so you can visit several islands.

Go see a travel agent!