Hummm.. Andorra in Summer ? Well, the main activity is hillwalking, plus some more hill-walking and the occassional bit of hill-walking.
Until a couple of years ago Andorra was dead dead in Summer with a few hill-walkers who liked the relative emptiness of the place and the very high peaks all around -mainly the northern part - becomes flatter heading south.
Now a few companies have added mountain-biking and adventure activities (canyoning,abseiling etc).
So, unless you're into that sporty fitness thing I'm unsure if you'd really like Andorra - no real historic or pretty villages ( though that's a personal view). It's not that close to an airport -Perpignan, Barcelona or Girona which are all closer to mountains, scenery,seaside and many pretty and historic towns and villages on either side of the border. (Plus you can visit Andorra on a long day trip from these places.)
However, I'm prejudiced and maybe you should contact www. hola-andorra.com who offer all sorts of accommodation and activities.