If you have not yet had chance to see these films, I really would recommend them! Spirited Away is a good entry level film and was presented in the West by Disney (though don’t expect Disney content!), whereas Princess Mononoke is a much more raw/traditional film I feel. Both are excellent.
The soundtrack of Princess Mononoke is especially wonderful. If you have surround sound at home, make sure that you use it, as the sweeping soundtrack absolutely fills the room!
Symphonic suite of music from Princess Mononoke, conduced by Joe Hisaishi (who scored the film)
Oh, and I want me a kodama (obviously!) – though the Studio Ghibli kodama do differ slightly from folklore kodama (these look more cutesy heh)