I think it's safe to say that most music students take it easy during the summer. Most music teachers like to as well. However, with fall just around the corner, it's time to strengthen those fingers!
I recommended a few popular books with hand exercises a while back. Last year, I finally worked through the first book of Hanon exercises:
The New Hanon (link to my affiliate link)
This summer, I'm working my way through the Schmitt Five-Finger Exercises, op. 16 (affiliate link).
What I really like about the Schmitt exercises is that they are five-finger exercises. Sure, you're supposed to play them 15 times each, but it's really not that bad. I use the following pattern:
Five times legato
Five times staccato
Five times something else
That "something else" could be portato, or jazz triplets or two-note slurs. I do notice that it takes at least five repetitions of the pattern before my tone evens out.
If you're looking for a way to get your fingers back into shape before the fall, you can't go wrong with Hanon or Schmitt. It beats playing your Conservatory scales, chords and arpeggios every day.