Completely disagree with the comment about Karachi.
There's beautiful golf courses, a number of very nice beaches - you need to find a friend who can book you a private hut and I'm sure someone on defence.pk will oblige if you need it. There's WONDERFUL restaurants. The Aqua Lounge in Zamzama is one of my favourites. The hand made chocolates also sold in an old Parsi shop in Zamzama are insanely delicious. Then there's the shopping, there's qawwalis at Abdullah Shah Ghazi's tomb, wonderful book stores, the world famous Bar-B-Q Tonite which is possibly the world's largest BBQ restaurant (not to mention, one of the best!). There's also the new Mohammad Bin Qasim park by the water and the brand spanking new food street which is Asia's largest. It's called Port Grand and has wonderful art shops and a cinema too.
If you like seeing old buildings, then there's Frere Hall, numerous beautiful churches... and you can't miss Mohatta Palace which houses an incredible museum. There are exhibits outside as well as on the grounds of the Palace (really a very rich businessperson's house, made in the 19th century), which has been lovingly restored to its former glory. Still on museums, the PAF museum is VERY impressive and has a great collection of aircraft. Not to be missed.
The art scene is Karachi is fantastic. If you like horses, you must watch a Polo match there. Some of Pakistan's most famous horse lovers reside in Karachi. There's horse races there as well (as there are in Lahore).
Depending on your budget, there are a large selection of hotels in Karachi. My personal favourite is the Sheraton. The last time I stayed there I got a room overseeing the grounds of Frere hall. It was very nice, and the service at that hotel is pretty darn good. They will arrange a car for you and will accommodate you in almost every way.
And oh, I almost forgot! The live music scene is hot these days. If you visit, you have to check this place out... T2F. It's a coffee house where there are regular live music shows, stand up comedy shows and much more. You can visit their website to view a list of updated upcoming events. There's always a bunch of interesting and intellectually stimulating activities going on:
http://www.t2f.biz/category/events/
Also, Karachi is Pakistan's most cosmopolitan city - it truly is a miniature Pakistan. You will find every ethnicity, every religion, every background. It is an unbelievable melting pot for sure!
This was a quick little intro, but you should take a look at this page on Wikipedia. It really has some very good travel tips and advice on places to see in Karachi:
http://wikitravel.org/en/Karachi