Churches can be visited at any time, provided their door is open. Most of the time there will be nothing going on inside, but you are welcome to look around (not to take photos or videos!), to buy a candle and light it and stand it in one of the recepticles, Bare arms and legs are frowned upon so take a scarf or something to cover yourself up. Most churches have a collection box and you could do worse than donate your small change in exchange for the opportunity to look around one of these beautiful old buildings.