One lens that I’ve been looking at, but don’t own yet, is the EF 28-300L IS. I realize that you have a concern about weight, but if you’re going to capture the subjects you normally get with the 135, the 24-105 will require cropping. As long as you’re test-driving lenses already, why not try that one?
I decided to do just that - try out the 28-300mm f/3.5-5.6L
IS. I think it will work well as a walkaround lens on my 5DII. Not worried about weight - I'm used to carrying a gripped 7D + 100-400mm for hours on a Blackrapid strap, plus, it's lighter than my current 'walkaround' combo of the 24-105 and 70-200 II, and it means no lens changes. I've got a trip coming up, planning to take the 28-300, 35L, either the 16-35L II or 135L, and a 430EX II. For me, that's packing light.
Personally, I don't believe in renting to try out a lens - I'd rather put that money toward ownership. Instead, if I'm not certain a lens will meet my need (which is the case here - concerned about my satisfaction with IQ from a superzoom, even though this is arguably the best superzoom available), I buy it used. I only buy from Craigslist, and won't pay more than 70% of the current new price. I did that for the 28-300, and that's with the current rebate - tomorrow when the price goes back up, I'll have an extra margin. This way, I can try the lens for an extended period, and if I decide not to keep it, even if I end up taking a small loss on the sale it would be less than the cost of a 4-day rental (and after having re-sold four of the five used lenses I bought previously, I've actually made a net profit).