Hell is very good if you like short, bouldery, 25-40 deg overhanging routes. It is also very much like an out-door gym, with everything that implies. The rock is some form of conglomerate that gives comfortable holds that are surprisingly hard to hold on to.
There is a small bouldering/trad/sport crag inside city limits called Korsvika (I've never been).
The best climbing "close" to Trondheim is probably in
Flatanger. The big cave in Flatanger is one of the most extreme crags I've been to. Over 100 m high featured granitic gneiss, more than 50 m overhanging.
In Vingsand, on the same peninsula as Flatanger there is some really good bouldering. Harbak is also supposed to be really good. Alas, I have not been to any of the bouldering areas around Trondheim.
The only multi pitch area I know of close to Trondheim is "Selnes", with bolt and trad routes up to 4-5 pitches. I have not been there either.