Inkarnate lets you draw your own continents, complete with features like forests, towers, and notes. You create an account, which is quick and free, and then you get access to this map-creation station:
You can then save and export the maps you make. Here's a little island I whipped up as an example:
That's pretty easy, but it gets even easier: the Polygon Map Generator will randomly generate islands for you. Then you can view them in 2D or 3D, plus from other interesting perspectives, like "Watersheds." You can keep playing around with the settings and randomly generating islands until you get one that inspires you. No login required - just hit the "Random" button! Then you can export islands you like.
Have fun exploring!