Originally posted by netuser Interesting, ... Why ?
Because you stated the vacation itself is the primary focus, and photography second.
The 17-50/2.8 is wide and fast enough for most situations, indoor or out. I suspect you won't find as much need for a longer lens as you might think, that you can certainly have fun and get great photos without one, and that the IQ and MP of the K20D will allow you to crop out some faux telephotos from the 50mm end of the 17-50 anyway, if need be.
Taking just one lens lets the focus remain on enjoying the vacation and not letting your photography get in the way of others with you enjoying it while they wait for you to monkey around with lens changes. Nor do you or they have to carry around all the gear that you think you
might use on a particular day. Remember: What is a passion for us can quickly turn into an annoyance for our loved ones....especially if they have to 1) help carry stuff, 2) wait around for us to reconfigure our gear, or 3) listen to us whine about how heavy all that unused crap is halfway through the day.
Hauling too much stuff around just turns into a burdensome chore and detracts from your enjoyment of the vacation. I would just mount that lens and put a spare battery and SD card(s) into my pants pocket.
I've never counted, but I would bet we have more "I took XXX and didn't use it as much as I thought I would" post-vacation comments than we do, "I wish I had also taken XXX" post-vacation comments.