Getting to and from Culebra is pretty easy if you plan ahead. The easiest ist of course, to fly, but then again you need to get a hold of the airlines. Then there is the ferry, which runs all the time and is really cheap. Then again, there might be the wait, if it's a holiday weekend.
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The truly fortunate charter their own plane. If you are a party of 6 to 10 this option actually makes sense fiscally as well! I have done it all, except the chartered plane. Taking a público from Río Piedras to Fajardo Playa is a pain if you have a lot of lugagge. But if you travel alone with a small backpack, it's the way to go!