Age 13-15? I'd have a few improving activities to break the holiday up interspersed with plenty of down time.
I would generally have one or maybe two separate weeks blocked out within the summer with a sports camp - not two consecutive weeks though - these camps are pretty full-on. It's good for them to get some fresh air and exercise and have a sporty week with other kids - mine would always be booked in with a friend though.
If you're having sone time off work, I would also use this time to invite a friend with her kids of a similar-ish age for lunch and an afternoon in the garden - it's good for them to help host a bit - maybe make the lunch together - make a quiche or something. It's a nice chance to catch up with family friends whose kids are at different schools.
Perhaps encourage them to plan a day trip away with a couple of friends involving a train or bus ride to a nearby theme park or zoo or similar. Encourage them to research it all online - phone to confirm details etc. great for their confidence to feel independent.
Realistically though, life can be so busy/ structured/ pressurised during term time that I think it's really, really vital that they have lots of down time so plenty of "nothing planned" days just to have big sleeps, mooch about, watch a bit of drivel on telly, read, play on the computer and generally veg. I found that when they've had enough rest they then have the time to do things like re-arrange their bedroom furniture which leads to a big clear-out but motivated by them having the time, energy and inclination rather than having to be nagged.
My older teens have part-time jobs and they've been able to up their hours in the holidays. They love working and earning money and have been able to pay for a holiday away with friends with the proceeds. Other than this, they are partying hard or sleeping!