Both a tailwheel endorsement and glider certificate will make you a better pilot.
The toughest part of flying a glider is keeping a good attitude on tow and not jerking the tow plane's tail around. Wake boxing can be interesting too. Landings are a snap, and if you have a lot of powered time, it's real easy to pick a landing point and grease it on.
Taildragging is what real flying is all about. If you have the balls to do it, your stick and rudder skills will pick up immeasurably. Your first close call (or actual) groundloop will scare the shit out of you, but you will get a lot of confidence.
Neither one of these is real easy to come by anymore, but if you have the opportunity, don't pass it up.