If I were to carry another lens it would be the Canon EF 50mm f/1.4 USM, or the 50mm 1.8 I am considering these lenses at the moment and might just buy one.
The 50mm f ocal length puts it nicely between the 35 end of the 16-35 and the 70 end of the 70-200. The wide apertures mean you can shoot in super low light. The lenses are so small it wouldn't be much to carry around. They sell for $300 and under $100 respectively,. There is a lot of discussion over which is better...I'll let you know my personal option when I get a chance to evaluate both for myself..,