As a native of Salisbury, brought up in the shadow of Old Sarum, I thought I should chime in. Stonehenge is an obvious target, but try to get there either early or late - it'll be crawling with tourists on the high-speed London-Bath round trip. If you're heading there by car, I would urge you to take the smallest roads you can navigate - you'll find a slice of 'real Wiltshire' around every corner. The Salisbury-Devizes road gives great views over the Downs. Salisbury Cathedral is also popular, but deservedly so. If you go there, make time to look around the entire Cathedral Close, the walled area surrounding the cathedral - all of Salisbury's best medieval architecture is there. It's worth bringing a macro lens and tripod as well - there's enormous history in every corner. Finally, if you're in the area of the Cathedral, it's an easy walk to the Town Path, which leads across the watermeadows behind the Cathedral - the view famously painted by Constable.