Airplane to Helsinki then either fly further to Turku or take an express bus. Bus 8 leaves from the marketplace to Ruissalo. Ofcourse you could also cycle there or walk.
All seasons are beautiful. The light in both winter as summer is stunning. The blue looks intense and in winter sometimes translucent. The nature is breathtaking and in winter time it is pretty easy to spot all kind of birds. If you love nature you love Finland.