As you say, dont waste it all, I have only recently finished hour building and just started the CPL, I was advised to practice much of what you have all ready done in PPL, and after starting the CPL it helps.
For the nav ex, practice planning and flying routes with legs around the 40nm ish, practice flying holding a heading an time, fill out logs accurately, dont feature crawl using landmarks, the routes have to be flown as if you had a paying pax in the back, ie the quickest way..once over your 1st detination, divert yourself somewhere again around 40nm, get a line on the map and head off, then start calculating dist, ETA.
Practice stalls, steep turns, flying slowly, etc, all stuff your familiar with really, just try and sharpen everything up. Use the time going through controlled airspace, matz crossing, and sharpen your RT.
Most flying schools have some sort of training book for hour building, giving you set exercises to accomplish on your own.
Have the odd jolly though, nice to just have a break from it all, also think about getting the 300nm cross country done, thats around 4 or 5 hours gone.