I find ITA Matrix a bit unwieldy but google flights is really good for most things.

You can can pick a few starting points as well as multiple destinations. Scandinavia seems popular but try AMS and BRU. You can also pick some different destinations and Oneworld only so BA plus AA domestic connections.

Its user friendly so im sure you'll pick it up quickly. If you are more adventurous than me you can graduate to matrix later!