Marlow GP, on the best days I have seen Kingfisher x 4, Hobby, Sparrowhawk, Green woodpecker, GS Woodpecker, Snipe, Little Ringed Plover, Lapwing, Terns, Tufted Duck, Egyptian Goose, Barnacle Goose, Shovellers, Water Rail, Long Tailed Tits, Shelduck, GC Grebe and chicks, and many many more.
For updates see the Bucks Bird Club Site,
http://www.bucksbirdclub.co.uk/menu.htm
and check latest sightings here
http://www.goingbirding.co.uk/bucks/birdnews.asp?date_search=8&date=2010-09-22&sort=7&status_id=8
On not so good days, Geese, Herons and Cormorants. Like any place dependent on thoughtless birds with no idea of their responsibility to provide a spectacle for us to view there may be nothing except a pleasant riverside or lakeside walk. I go at least once a week.
If you want to see Kites, lots of Kites, head to Stokenchurch up the M40 and pull off at J5. Dozens of Red Kite in that part of the sky all day every day.
Good luck.
obm