Sensoji is a Buddhist temple located in Asakusa and is one of Tokyo’s most colorful and popular temples (source: japan-guide.com). It is easily reachable through the metro system (Asakusa Station, exit 1), it has free admission and is well worth a couple of hours of exploring.
The temple was completed around year 645 and is the oldest temple in Tokyo, build in honor of Kannon, the God of Mercy. It suffered major damage in WWII, but was reconstructed afterwards into its former glory. As you approach the entrance to Sensoji temple, you will pass a Nakamise Dori, with lots of shops selling crafts, souvenirs and plenty of sweet and savory snacks.
Even if the entrance into the area is free, you are welcome to donate money or to by a Omikuji Paper fortune. If you dare buy a fortune and end up with a bad omen, rest assure that not all hope is lost. There are ways to neutralize any bad fortune you may be predicted.
As always when visiting religious, sacred grounds, be respectful and curious and you will have an excellent experience.