It isn't exactly Pebble Beach, but there is no shortage of exotic challenges on the Colby Golf Course. And it has hazards the greatest course designers never imagined--buildings, for example.     Campus golf--typically played with a tennis ball, a driver and a putter--has been around informally for years, but this year I-Play got serious (if that's the right word), producing a playing manual complete with photos of each hole and tips on how to approach them.     Designer Jake Conklin '00J used peculiar features of the campus to create a mixture of playability and goofiness. On the par 4 fifth hole, for example, golfers tee off from the gates between East and West Quad and finish by hitting the dumpster behind Miller Library. A major impediment to completing the hole is that a car could run over your ball as it rolls across the semi-circular driveway.     Traffic also comes into play on number 13, which traverses the quadrangle below the library, crosses Mayflower Hill Drive and ends at the steps of the Lunder House, where unsuspecting recruits and their parents present additional concerns.     The signature hole is 18, a par 5 that tees off next to the statue on the Bixler patio, passes under the Arey-Keyes walkway and finishes at the flagpole in the center of the campus.     The course description is mute on what constitutes "out of bounds," so our advice to campus pedestrians is the same advice golf instructors give to their pupils: Keep your head down.