La Sirenita on Eastern Ave in Highlandtown has lovely horchata and great tacos. just had Mari Luna for the first time recently and the tacos and tamales were spot-on! Didn't try the horchata, though...
i know that taco fiesta near spinnaker bay has horchata, but i have never had it...however, while it looks like a chain restaurant, it isn't and their food is really good.