Colombia and Peru

I myself am planning a Colombia trip, but can give some info on Peru. If you only have a few weeks, you won't have much time to see anything. I suggest limiting yourself to one of the countries, and a specific area of such country at that. I spent seven weeks in Peru and there still was quite a bit I did not see.

If you like to see ruins and history, there are plenty to see in northern Peru. While the area is accessible, not many people go there. Some cities that come to mind are Trujillo, Cajamarca, Lambayeque (the Bruning Museum) and the larger city a stone's throw away from Lambayeque whose name escapes me at the moment. You'll need to be a Spanish speaker, as you won't see many foreigners around.

Peru?

I liked Lake Titicaca (Pune, or Copacobana on the Bolivian side), and the Colca Canyon (Arequipa). Nazca was good to see the Nazca lines, and the coast is pretty. I would give Pisco a miss.

Colombia?

Depends on what you like.

I live in Bogota, and find little to recommend spending more than a few days.
Cartagena is the biggest tourist attraction for good reason (very pretty). Apparently it is wild for Xmas and New Year.
Villa La Leyva is pretty, but not mind-blowing - skip it if you don't have much time.
Parque Tayrona is fantastic (for nature, obviously).
Gorgona, I am told, is also great (for nature)
San Agustin is supposed to be out of this world.
Medellin and Cali are nice cities
Barranquilla sucks
Santa Marta is a bit dull, Taganga is okay.
San Andres is pretty but a bit tacky
Providencia is, well, paradise, but everyone's tired of me going on about that...

In terms of New Year, it would have to be a city. I would vote Cartagena, closely followed by Medellin/Cali.

1) Definitely a Colombian city. I thought Bogotá had better nightlife than Cali or Cartagena personally, though I know this is not the general consensus, and I didn't go to Medellin. Colombians are rightly famous for their party spirit - I particularly liked Kukumukura in Bogotá, and though it's expensive, Andres Carne de Res (in Chía, just outside Bogotá) is a real experience and a half.

2) The most interesting way between the two would be by using the Amazon. between Iquitos (Peru) and Leticia (Colombia) or vice versa. Neither one has land links with the rest of their respective countries, but Iquitos has river connections with other parts of Peru from where you can connect by road. The river journey is 400km. I haven't done it myself, but my experience of the Leticia-Manaus river trip was almost entirely positive and I'd definitely recommend Amazon riverboat as a mode of transport.

3) Personally I hated Peru. There are some incredible things to see and do there, Machu Picchu and Nazca to name two, but in general I despised the people. I felt everyone was always trying to rip me off or steal from me - I was just a walking dollar sign. Ironically I write this from Venezuela, where I am feeling pretty much the same. Colombians, however, are in my experience much more welcoming, hospitable and friendly - I loved my time in Colombia and will definitely return. Speaking at least basic Spanish is a must though. In addition to the other suggestions, I really liked Salento, a pretty little town in the Colombian coffee country, which has good hiking around it.