One of the first PC games I ever played all the way through was Twinsen's Odyssey, or Little Big Adventure 2 as it's sometimes known. I was 9 years old, and it was one of the most fun games I had ever played at the time.
I never played the prequel, but the sequel didn't really require you to have played the original. It was open ended, and a lot of fun... with a good mix of platforming, action, and adventure, mixed with unique play styles.
The most remerable part is when you get stuck in the middle of an epic battle between an army of giant flies, and talking sausages. I'm not making that up.
They also had guns, you had a... uh... bouncy ball.
It also had a flying dinosaur in it... yay!