This is an unusual temple, in that the attraction is the statuary inside. There are 1001 statues of Kannon, an East-Asian Buddhist deity who has 11 heads to understand the suffering of all creatures, and a 1000 arms, each of which saves a world. The statues, which are over 700 years old, depict the same image, minus about 970 arms, but each has a slightly different shape. There is a large image of Kannon in the center. The Kannon statues get old kind of quickly (plus they really need to be dusted), but there are 28 statues of guardian deities. The guardian deity statues are always superbly executed wherever they are found, not just at Sanjusangendo. But here there are a huge collection of them, all being excellent depictions of human form in action. The guardian statues are magnificent, and are definitely worth a trip. For those wondering where the sanjusan (33) comes in, there are 33 spaces in-between the pillars of the temple.
You can also see a bit of the workings of Buddhist temples here, since there are English translations for everything. You can purchase candles, which you can write your prayers on; Kannon presumably becomes aware of them when they are burned. You can also write prayers on small, specially-shaped pieces of wood, which are burned during a special ceremony. You can also buy incense to offer. Japan is a very trusting and low-crime country: everything is done on the honor system. Although, there are 1001 Kannons to see you steal an candle, so you probably wouldn’t get your prayer answered that way...
Sanjusangendo is about a 15 minute walk from the Gojo Guest House, and probably about 20 minutes from Kiyomizudera.
You cannot photograph the sacred Buddhas, so you will have to buy some professionally done postcards if you want a record of the statues.