I'm not doing a particularly good job on the strengths - what kind of things can you say about how wonderful your group is?
Do you have external people you can ask for perceptions of strength? For example, visiting scholars: why did they choose to come? Friends you are going to run into at conferences - can they identify three strengths in return for a pint next time you meet up at scientific meeting on terribly interesting topic? At my former U were often astonished at the very good reputation of our department among outsiders - but when they explained why it made sense. Being on the inside every day meant we were aware of the problems and downplayed the positives.
As a starting point you might try to identify particular areas of expertise - particularly if they have been identified as world class. Do you have specialists that are in demand for talks/services? They could also be regarded as a strength. Do you have a unique degree program that your research feeds into? .......