It’s difficult for me to understand what you are looking for without a little more information. “Not too hard” is always in the eye of the beholder, and what technique a piece will help with depends on your individual skills. What were the last pieces you played? How long was your break?When I had a many decade break, I started back with revitalizing the music I could play before the break and then began working with a teacher on the approach I should take. Just a thought.
All pieces are ‘technique strenthening’ If you need to learn the skills to play the music Saying you play a couple of Chopin prelude is not helpful, because they vary in difficulty a great deal. I would suggest learning them allLook at the Debussy that was mentioned Add a couple of Bach prelude and fugues Can you get a teacher? That would really help get you on track and keep you there
I'm not a musician, but for what it's worth I'll list a few pieces my (intermediate level) son has played recently that I loved:Chopin, waltz in A minorLiszt, Cosolations 1 through 3CM Weber, Sonatensatz, TrioOut of the many Bach preludes he played no. 936 is my favoriteAll of these should be easy for you, and they are lovely. Have fun!