Nagoya University WebCJM Japanese Grammar Online has a 20 Lesson selection of quizzes covering basic Japanese grammar. Based on the book A Course in Modern Japanese (CMJ) [Revised Edition] they look like this:
Lesson 12 Exercise 1
Change the form of the verb into potential forms:
Very dry and jargon-heavy but if you ever want to teach Japanese or win arguments over the difference between perfect and perfective then highly useful. Seriously, though not by any means exciting this is good solid stuff.