If your trip has some time built in beyond the houseboat itself, a handful of spots are worth working into the schedule.
Vembanad Lake - The largest lake in Kerala, and the backbone of the whole backwater network. Most houseboat cruises pass through part of it, and it's especially calm early in the morning.
Alappuzha Beach - Right in town, with an old pier stretching out into the Arabian Sea that's been standing for over 150 years. Evenings here get a proper crowd for the sunset.
Alappuzha Lighthouse - Built in 1862, still functioning, and open for visitors to climb up for a wide view of the coastline and the backwater belt behind it. Sits close enough to the beach to cover both in one stop.
Pathiramanal Island - A small island in the middle of Vembanad Lake, reachable only by boat, and known for the birds that migrate there through the cooler months. Worth it if anyone in the group is into birdwatching.
Krishnapuram Palace - Around 45 km from Alleppey town near Kayamkulam, this old palace has one of the biggest wall paintings in Kerala. Most tourists don't go here, and that is why it is not crowded at all.
Marari Beach - A peaceful beach area not far from the main town. It is great for travelers who want a beach experience away from the heavy rush of Alappuzha Beach.
Nehru Trophy Boat Race - If your travel dates match the second Saturday of August, planning your days around the snake boat race at Punnamada Lake is a great idea. It is one of the main festivals here and a lot of people come to see it every year.