I was in girl scouts from 1st grade all the way through high school (sometimes reluctantly). I'm also a lifetime member and spent one summer working at Our Cabana (Girl Scout World Centre in Mexico) back in 2002. I think breaking for the summer is one reason why older girls typically drop out - they lose that connection with each other for the summer and then it can be very awkward when the year starts back up. Summer is a great time to go to camps or do interest projects that involve summer activities, like swimming, camping, etc. You don't have to meet every week, but once a month or at least once during the summer to keep everyone connected. It's too late for this summer but I would suggest that you start planning now for next summer. Make some sort of plan (whether it's a lock-in at the Y or museum or some such event) and set the date early in the new school year. That way, it will be on people's calendars very early and encourage parents to make vacation plans around this date. Plus, this gives the girls something to look forward to all year round and raise money for it (if necessary) through cookie sales and other things. I think the same is true of brownies, although I wouldn't plan any summer activity that's too intense for these girls - something age appropriate is better.
Girl Scouting in other countries really flourishes, but it just kind of fizzles in the US and I think this is one reason. Scouting is a lifestyle, not just a once a week or once a month activity.