After explaining to students the need for assessment to be valid and meet the other requirements suggested here, you can ask the students what kinds of assessment tasks they'd find rewarding to do. Variety of assessment tasks may look after itself. If the learning outcomes are varied, then the appropriate assessment tasks will also be varied. However, if the learning outcomes are similar in kind, then extra effort may be needed to achieve variety. The ideas described below about types and styles of assessment tasks should help.
Efficiency is measured by comparing outputs with inputs.
Note how long it takes you to set assessment tasks, if appropriate to administer them, and certainly to mark or grade them. This is useful knowledge. Most of what students put into their assessment is their time and effort. The module description will say how students should be spending their time. Since the primary purpose of higher education is student learning, assessment tasks which take a lot of student time are fine as long as they generate positive motivation, useful learning activities and learning.