Abbottabad is a transit town more than a destination, but it has enough to fill a day if you are stopping over. The most rewarding things involve getting out of the city for a few hours.
In the city itself, the British cantonment is the historic core — quiet streets, gardens, and the surviving colonial buildings. St Luke's Church, the Old Christian Cemetery, and the Jalal Baba Auditorium are all walkable from the centre. Shimla Hill offers the best view of the valley and is reached on foot from the main bazaar.
Out of town, Thandiani, the Galyat hill stations, and the Ilyasi Mosque at Nawan Shehr — famous for the cold spring that bubbles up inside it — are the main half-day options.