1. Rent a car for two days. You can do this right at WDW.
2. Take a shuttle service. Mears offers one and I believe its $18/pp roundtrip.
3. Take a taxi. We did this years ago and as I recall it was around $25-$30 each way.
There's also a public bus system (Lynx) which is really inexpensive (around $2) but I don't know anything about it other than what's been posted on these boards and others. It sounded pretty complicated and time consuming to me.
Personally, if you're going for two days I would rent a car. I like being able to come and go as I please. In fact, that's what I've done for my upcoming trip. We're staying at US/IOA and doing a day at WDW.