My 'dream team' for accompanied vacations like yours is a pair of bodies (E-510s in my case) and a 9-18 and 18-180 combo. I also carry the 'shirt pocket' XZ-1 for use in the evenings as it is a joy to have at least some time free of being festooned with gear.
You probably won't use the extreme telephoto end much in the urban setting so the 70-300 won't offer much over the 18-180 in terms of functionality.
I'm aware you don't have the 9-18, but why not splash out on one? The cost is presumably nugatory compared with the cost of the holiday overall.