If you are looking for a solid, basic towable tube, Airhead offers a great selection.  Ranging in size from a simple one person round tube, up to a multi-person chariot style rideable - Airhead has you covered.  They offer an especially nice selection of "youth towables", small cockpit style tubes especially suited to the younger kids.  And they're probably the most affordable towables around.  Bright, vibrant colors add to the fun.