Gangaramaya Temple is a busy place, architecturally influenced by a whole host of styles. The grounds, close to Colombo's Biera Lake, are populated with a bodhi tree, image house, relic chamber, pagoda, and a monastery.
The museum itself is a busy room filled with artefacts collected and received as donations over the years. You can find souvenirs of British Kings, large thrones and elephant tusks. It's an interesting and engaging experience, but if you can get a guide or a friend to show you around that can be more illuminating.
There's also a shrine at the nearby lake which was designed by much revered Sri Lankan architect Geoffrey Bawa.
Make sure you wear long skirts or trousers, cover your shoulders and take off your shoes when you visit (this is a mark of respect expected in all Buddhist temples in Sri Lanka).