Android's version of C#'s Console.WriteLine?

Check out the help pages for Android.Util.Log.

You can use:

Log.v("MyActivity", "Verbose output");
Log.d("MyActivity", "Debug output");
Log.e("MyActivity", "Error output");
Log.i("MyActivity", "Information output");
Log.w("MyActivity", "Warning output");
Log.wtf("MyActivity", "WTF output"); // This isn't a joke :)

For simple debugging, the standard Java way suffices:

System.out.println("Message");