My vacation kit fits in a small Canon Backpack (fits any airline carry-on underseat rule). I take a 30D with battery grip and hand strap, 16 gig of San Disk EX III cards (two 8's, about 2,000 RAW images), 2 580EX flash units with fresh charged batteries (Or one 580EX, and my SD800 with the Canon underwater case in the same space) , one EF-S 17-55, one EF-S 10-22, one EF 50 f/1.4 (or an EF-S 60 f.2.8), a 1.4x extender, two extra camera batteries, twelve freshly charged AA batteries, two charged batteries for my SD800 IS, a remote timer and extension cord, and either the EF-70-200 f/2.8, or an EF 70-300 DO lens depending on the venue. I like the EF 70-200 f/2.8 for most situations, and sometimes pack the DO lens in my checked baggage anyway. If I take the 70-200, I carry the Canon extender. If I take the 70-300, or both, I take the Kenko extender.
A light monopod works well if carried in checked baggage, or you can buy a light, cheap tripod when you get to your destination, and give it away before you leave. I have done that several times, and it works like a charm. If you are going to a destination where a proper tripod is essential, pack your favorite in a soft case, and send it as checked baggage.
When you are far away from home, and not likely to visit there again real soon, bring the best variety that you can fit in an underseat sized bag. Don't compromise more than you must. If you have never been there before, you want as many tools available as you can reasonably carry. You can always leave some gear in your room when out in the field, but you simply can't get your hands on more gear if you need it.
Sometimes, I stick a macro lens, and the MT-24 zipper cased Twin Lite in my checked baggage just to have it, even though macro shooting isn't a priority on a vacation trip.
Everyone has their own preference with this sort of thing, and I understand the motivation for each response. I just try to pack as many tools as is possible within the dedicated space of a backpack that fits under the seat in front of me. You would be very surprised as to how much gear you can carry in one of those $35 Canon packs....
No matter what you take, have a great trip. BTW, don't leave home without a good P&S camera (with a cheap factory underwater housing if surf is a main feature), and don't leave home without insurance. Enjoy your trip!.
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Voyager