Rancho Notorious is the only ranch in Aruba where you can ride along the water's edge on the beach for quite a ways (north of the Marriott). We've ridden with them many times over the years and have always had a great ride. They will ask you about your riding ability and match you to a horse suitable to your experience (or lack of.
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http://www.ranchonotorious.com/
Another ranch that we like quite a bit is a small mom-and-pop operation called The Gold Mine Ranch. They have very few horses so the chances of you getting a "private ride" are pretty good. We rode with them last Sept. and loved it. Hope to ride with them again this year.
http://www.thegoldmineranch.com/