Anytime you can get students to practice grammar in a different way especially in game form more learning happens than just straight drill. The concept of when you use the preterite and the imperfect in Spanish can be a hard one for students. One of the best ways is just to practice deciding which tense works in different types of sentences.
I have a simple game using Mini File Folders and cards cut from cardstock (two inches by about one and a half inches) that I use with my more advanced students to work on the concept of the two past tenses. Each student needs a mini folder (these can also be made with big folders, but I like the compactness of storing this game with the mini size). In each folder you have a side for the preterite and for the imperfect. You need to draw an outline of about the size of the cards you will be using. Each side needs five outlines (see the above picture). The goal is to have a spot for each person/verb form in the preterite and the imperfect. So for example on the left hand side you would have in the preterite these spots:
Él, Ella, Ud.
Ellos, Ellas, Uds.
Then you need the same on the right hand side for the imperfect. You then need to create cards with the verb not conjugated that would either take the preterite or the imperfect. Here are some examples:
Nosotros____________(ver) un accidente.
Todos los lunes Carla______________(ir) al cine.
Yo estudiaba cuando ella____________(llamar).
Ayer Lisa y Carmen no_____________(hacer) la tarea.
Create enough cards in the pile so that each student could fill his/her file with an example of each (basically so they could cover all their spaces), but then create some extra cards.
Here's how you play:
Students take turns picking up a card and deciding what tense to use. They then say their sentence outloud conjugating the verb correctly. They place the card on the correct tense and person in the file folder. If a student picks up a card for a space they already have covered it is like missing a turn, and they place the card at the bottom of the file. The object of the game is to fill your folder first by covering all the spaces.
This game can be used with other concepts in the language like ser vs. estar.