While the easiest girls are often found in Bogota and occasionally on the coast, some claim the best-looking girls can be found in Medellin or Cali.
If you speak Spanish, then Periera is an excellent option for a quick trip.
It's hard to go wrong in Colombia, as long as the city has a good population. Don't confront Colombian men or try to take their women.
I've talked more about the women in Colombia here: I spoke in-depth about the cities in Colombia tailor made to meet Colombian girls.
We both were looking for no-strings-attached affection.
Casual sex isn't all that great without some level of emotional connection and attachment.