If you've not been to Europe before I'd recommend London. Lot to see & do there, plus day trips to England locations as well as a quick hop to the continent for a (long) day trip, and they speak English.

Having said that, given that Hawaii & Europe are totally different what exactly would you like to do during your 7 days. That probably makes a difference on what we recommend & why? The more information you provide us the more we can make more targeted/informed recommendations.

Cheers.