I presume that you are looking at an evaluation system which breaks away from the traditional question format used all across, to test the conceptual understanding of students. Your question is very pertinent purely because the very thought of grammar brings about a very negative thought process in the students mind. As teachers, our endeavour should be to make students like the methodology we use to test their understanding and more importantly they should look forward to such kind of assessments every time. 

I suggest that you go in for a combination of various methods, as in, multiple choice questions, fill in the blanks, error correction, synonyms, antonyms, using idiomatic phrases, phrasal verbs and foreign words in a sentence and then asking the students to write down the meaning without giving them any choice. Obviously each module will require a different methodology to make the exercise interesting and challenging. We cannot have a common evaluation system to test all grammatical concepts, which you will agree.

You just need to sit on this and then putting yourself in the place of the student, think about the methodology which will challenge their understanding and also retain the challenge. At the same time, also ensure that the methodology you use to explain each grammatical concept to the students, is also interesting. You need to synchronize both these parameters.