4 day trip from BWI/DC
 

Which island would you suggest for Memorial Day weekend? We want a direct plane ride less than 3 hours and a nice Beach.

Turks Caicos Islands have great beaches and should be a direct flight from BWI.
Bermuda is very nice and should also be direct.

We were in your position in January, and opted for Jamaica--an easy 3.5 hour trip on Air Jamaica. Negril is a great beach, and I highly recommend it. We would have considered Bermuda, except temps aren't warm enough for us in January.
I don't believe there is a direct flight to T&C from BWI.

I just booked a 4-day weekend (albeit in February) in Puerto Rico. The AA flight from BWI is non-stop and leaves early -- you are in San Juan before lunch time; the return flight doesn't leave San Juan until after 6. The flight is about 3.5 hours. You can really maximize your time that way.

Bermuda might be another option out of BWI, but I don't believe there are any non-stops. Last time I researched it, the most direct route was USAirways via Charlotte or Philadelphia. That may have changed.

From BWI, there are not many islands that are less than 3 hours away. In distance, the Bahamas might be, but most of those trips require a stop in MIA or FLL, which lengthens your journey and shortens your quality time on the beach. (I would, however, look into Air Tran; I believe they have a direct flight out of BWI to Freeport.) Any further south than the Greater Antilles (Jamaica, PR), and you are talking about a longer trip.

There are non-stops from Reagan National to Bermuda and to Naussau.

If you have the budget, Bermuda is interesting but expect to pay.

We flew to the Turks and Caicos from Reagan in Jan, connecting through Charlotte. Left at 7:30 a.m. and were on the beach by 2:30 p.m. Check out the reviews for the Sands Resort there. We really liked it.

My friends and I are going to Cozumel for Memorial Day Weeekend from DCA. We leave around 7:45 am and arrive around 11 am on Saturday, and leave Cozumel at 2 pm on Tuesday afternoon. We do have a short layover in Charlotte. There are some pretty good flight deals there I believe (but I used miles). I have been to Cozumel once on a cruise and loved it. We went to a beach club on Paradise beach, and did amazing snorkeling right from shore. We had planned on exploring the island more, but we liked the beach and snorkeling so much that we stayed there all day.

You might also look into the Bahamas-- I researched it for this trip and I believe there are some non stop flights, and it is closer than Cozumel. Turks and Caicos are fairly close as well. I have never been there but I have heard they have amazing beaches.