I'm an Oslo girl myself, but being interested in rock climbing and hiking in the mountains some of my favourite spots in South Norway are:

* Nissedal in Telemark county (a tiny, sleepy place in the middle of nowhere, but with some of the most spectacular granite walls in Northern Europe),

* the Jotunheimen mountain range (excellent for short or long hikes, unique self-service hut system, many peaks that can be reached in a 1-day hike: favourite tent spot - Riingsdalen valley in West-Jotunheimen)

* the train journey from Oslo to Bergen passes over some beautiful mountain scenery, and there's a well-known bike trip that takes 1 or 2 days alongside part of it (Rallarvegen, the road that the railway builders used)

* Bergen itself is considered a very picturesque town, but rainy as all get out.

* and I guess the fjords are a must for tourists, but coming from Norway it's a bit hard to see how impressive they are