I think for just pure 100% fun, you can't beat one of the fast sports zooms. I am also new to photography and while I had targeted the 70-200mm 2.8 zoom as an eventual must-have lens due to my interest in equestrian sports, I was surprised to find out how versatile this type of lens is; Wildlife (great for waterbirds), Portrait, Lunar images-it's been really great for all of these types of images.
I got the Tamron 70-200mm 2.8. It's much less expensive than some of the others and is super sharp and fast. I found it to be great for a beginner like me because it's hard to take a crappy picture with this lens. It's just pure fun.