It's hard to answer so open ended a question, but here are a few general comments from my experience.
Crete (and I speak as someone who uses alternative transportation whenever at all possible) is one place where you really need a car if you want to see it extensively. It's certainly possible to get almost anywhere most people want to see by bus (I've done it,) but it takes a lot more time and planning than going by car.
One question to ask yourself is, how interested are you in Minoan Crete. Of course everyone should go to the Heraklion Archeological Museum and the Knossos site, but there are a great many others which are worth going to if you're interested in that culture, but probably not worth it if you aren't.
Another question is how interested you are in hiking. Crete has some spectacular landscape which you can see from car or bus, but it's worh getting out into if you are fit for it. The one great outdoors attraction is the Samaria Gorge, which any guidebook will tell you about. Other walking opportunites abound, from casual walks around villages to strenous cross country hikes through rough country.