The Torre Escénica — an 80-metre observation tower at La Isla Shopping Village — gives a 360-degree view across the Hotel Zone, the open Caribbean, and the inland Nichupté Lagoon. A glass-walled pod climbs slowly up the tower and rotates as it goes, taking roughly 12-15 minutes for a full circuit. On clear days you can see Isla Mujeres to the north and the curve of the entire Hotel Zone in one frame.
The tower is open daytime and again at sunset (often the better visit) and admission is around US$15. Combine with a wander around La Isla mall or a meal in the canal-side restaurants below. Not worth a separate trip but a worthwhile add-on if you are already in the area.