au contraire! I'd suggest the best single lens to take with either of your cameras is the 28-135is for the following
reasons. It's relatively cheap to buy - around $300 - but has good enough image quality that it has been a standard package zoom with many Canon bundles. It's relatively cheap to buy so you're not going to break the
bank if it's damaged or stolen. It doesn't weigh as much as some of the alternatives suggested here and most vacations I've been on involve a lot of walking without many "rest stops". You can sell it after your vacation if it doesn't fit nicely into your long range lens plan and you won't lose much, if any, money. If you decide to take the
50 crop sensor, the 18-135 is another similarly priced option.
Have you shot with it? With a 5D3, I don't think the OP wanted to sacrifice IQ. If IQ is not paramount, then an SX50 or SX40 P&S will serve the OP well.
I had the 28-135, and to be honest its IQ and build are crap compared to the 24-105. Pardon my French
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses