For the AP Computer Science class I try to incorporate Gridworld throughout the year. Each unit we do at least a half day of lab excesses relating GridWorld to the material we just covered.
I want them very comfortable with using it. I have also found it is a great way to introduce topics. "Remember when we did that GridWorld lab and it used the word extends...?". It is a great way to give them a framework for some of the more abstract stuff we cover.
Here is my LabManual and the java files that go with it. BoxBug is from the student manual AP provides.
I have to say my favorite is the RockMonster - originally it was the RoachMotel (bugs check in but never check out), but sadly my kids didn't get the reference. I am sure the WS are "buggy" and are offered as is. I'll b updating them as the year goes on.