Heh, I will also be visiting Yellowstone in July (last week of hte month) for a family gathering... Blasphemous as it may seem to some, I will probably leave my Pentax kit at home (K100D and assorted consumer-grade glass plus the DA12-24), and take my little Olympus XZ-1 instead. I'm not out to capture close-ups of distant wildlife, but rather the scenery, family snapshots, and that sort of thing. The camera in your pocket and your hand beats the camera in the bag in your trunk a mile away...
And, with that rig I can take along my dive housing and not worry about dunking anything out in the lake on a kayak...
If I do take the Pentax, I'd probably take the 12-24, the 18-55 kit, and my 70-300. With the expectation that there'd be a lot of lens-changing (I say I'm not out after distant critters, but if I take the glass I'd end up using it and trying anyway...).
Jim