Well, I thought it was easy to walk out to the street when I wanted to head out somewhere, and the drivers were plentiful--begging for business (though it was the off-season). Picked and "interviewed" one for about 5 minutes, bargained a fare for the journey and return, and off I went. This was in Ubud...not sure about Nusa Dua but should be doable unless you are really in an isolated location. My preference is to put the money directly in the hands of the service provider rather than to the hotel that skims off a lot of cream, though some people feel more secure/safe with a hotel-provided driver. Don't forget to get straight--before you commit--who pays for fuel and driver's lunch.