The Collaborative International Dictionary of English v.0.48:

Sight-seer \Sight"-se`er\, n.
   One given to seeing sights or noted things, or eager for
   novelties or curiosities.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sightseer
    n 1: a tourist who is visiting sights of interest [syn:
         sightseer, excursionist, tripper, rubberneck]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "sightseer":
   Paul Pry, Peeping Tom, adventurer, alpinist, astronaut, busybody,
   camper, climber, comers and goers, commuter, cosmopolite, cruiser,
   eavesdropper, excursionist, explorer, fare, globe-girdler,
   globe-trotter, goer, gossip, hajji, inquirer, inquisitive,
   inquisitor, jet set, jet-setter, journeyer, mariner, mountaineer,
   nosy Parker, palmer, passenger, passerby, pathfinder, pilgrim,
   pioneer, pry, querier, querist, questioner, quidnunc, rubberneck,
   rubbernecker, sailor, scopophiliac, slummer, snoop, snooper,
   straphanger, tourer, tourist, trailblazer, trailbreaker, transient,
   traveler, traveller, trekker, tripper, viator, visiting fireman,
   voortrekker, voyager, voyageur, voyeur, wayfarer, world-traveler,
   yenta