As ofhers have suggested, acro training in a glider. Unless you have gyro instruments in the gliders you fly, instrument and under the hood training will be of little practical use, but it is fun.

Practice at landing at every kind of airfield. That means using the radio (while scratching down low - whats ATIS and tower freq?) to talk to a towered field and picking where to land (the big instrument runway is probably NOT the best place with an airliner inbound!).

Using radios correctly (hint: spend time reading the AIM). Flying a safe pattern at a strange field when you get there at 500' agl. What to do when a 50 knot gust front gets to the field when you are in the pattern (hint: land into the wind regardless of runway direction).

Think of possible emergencies and practice them with an instructor. Fly a no-rudder pattern to landing. Pull full spoilers on tow, release, then fly a safe pattern and land without closing them.

Kirk
66

Help maintain or fix a glider to understand how it works better.