well, if what you want are candid and natural shots, smile at the person and shoot. after you've gone trigger happy, if you want, you can smile at the person again or even go up to have a small little chat with them, tell them what you're doing, share something with them. be friendly
Haha that's where zoom lenses come into play. If you're too shy to ask, or are afraid that the spontaneity will be lost, position yourself some distance away, pretend to be interested in something else, when they're not looking or preoccupied, turn and shoot. Speed is essential for a quick and silent hit.
Ok it may sound low down but even professional photographers do it very often, especially photojournalists and travel photographers. The person may be annoyed but what can they do?
But don't try this with kids when their parents are around. Always ask their permission as parents tend to be very protective and may make a scene.