It's a bit unfair to say there is only a day's worth of things to do in KL. I had a perfectly enjoyable 6 days there last year.
It not Asia's funnest/ most interesting city, but there's plenty to do. I don't want to repeat, as El Regina made a very detailed list above, but the Bird Park was pretty special. I also enjoyed a trip to the Batu Caves too.
There was plenty of delicious food, at dirt cheap prices too (compared to the UK).
I certainly enjoyed my trip to Malacca, but there certainly isn't as much to do there as KL. Though I really loved the Malacca style Chinese food - which I didn't see (but is probably available) in KL.