Just got back from a trip to Europe with the family, four days in London and four in Paris. There has been a lot of discussion of compact kits recently, so I thought I would chime in one what I carried and why.
Here is what I had on me at all times:
The small Tamrac bag holds a IIIf with collapsed summitar, and M3 with 21mm SA, and a light meter (I had a second meter in my pocket). I had the divider arranged on a diagonal, and this combination fit snugly but comfortably. I kept B&W film in the IIIf, color in the M3. My rationale was that I was more interested in color shots of museum interiors and of architecture, and in both cases the wide lens would be better. The IIIf was for street shots where I prefer B&W anyway.
The small belt bag held a summicron DR and Canon 50/1.4. I sometimes switched to the DR for closeups and a few color portraits, and I sometimes substituted the 50/1.4 for the summitar, as it is very sharp when opened.
I want to attest that this worked out reasonably well - the little bag didn't break my back over ten days of pretty strenuous touring on foot, and my strategy was all-in-all OK; for a lot of cramped interior shots I would have had less interesting views without the 21mm, and I am comfortable with 50 for outdoors.
Drawbacks: The 50mm DR is I think my best lens, and it did not see much use with this strategy. If the 50/1.4 is on the IIIf, it really will not fit in the bag with the M3 (likewise there is trouble if the the M3 has the DR attached). Another problem is that on a few shots yours truly was moving quickly and forgot to extend the summitar! So I know I will be kicking myself for some "periscope" shots.
I shot 32 rolls over ten days, which I think is on the "low" end (~1,000 images, bet I would have had 5X that with a digital). I was the only film photographer I encountered throughout the entire trip, and I did have some envy of the ease with which the digital crowd was collecting images at a furious pace. They also were not propping themselves for a 1 sec exposure in Notre Dame (although I chimped over their shoulders and was not very impressed with the quality of what I saw). Somehow I felt right about what I was doing.