Interesting that there are so many "21 and 50" replies. That's a really nice combo, IME. The 50 is such a versatile lens, it can shoot anything; and the 21 is a great wide- used carefully it can do a lot too.
A few years ago, I took a week-long trip out of the country, and I agonized over what gear to take, wanting to cover all the bases I thought I'd need, but wanting to go light. I ended up taking a Leica M3, a 50mm Summicron, a 21mm CV Skopar, and an Olympus XA. This has become a very common kit to carry, whether for a day out or three weeks on the road. 21, 35, and 50. I could even shoot a wedding with this, if I had to.
I know this is a "which two" thread, and there are lots of great answers, none of them wrong; but after a few years of chasing around this question of what lenses for my kit, and reading a million threads of this nature, I have come to the (classically inevitable) conclusion that for me, the best Leica kit is 21-35-50-90. There's a reason these are the most common classic Leica focal lengths. These four lenses and two bodies are by far my most used lenses, and will cover 95% of the pictures many of us will want to shoot. With this in mind, it's perhaps difficult to pick the wrong two of these for a great two lens kit. Depending upon the size of your lenses, bodies, and bag, you might just carry three or even all four. Certainly my 21/50/M body with the Oly XA for a 35 makes for kit that's small enough to go anywhere.
Enjoy these responses, and I hope they give you some more food for thought; and enjoy your trip!