Can every activity you do be used as an assessment? If not, stop doing them. Every activity you provide for learning should be meaningful to you and the student; that is, the student should understand why they are doing it, and you should be able to ensure they did the work and the level to which they understood the concept. If these conditions are not met, the activity will waste your class time, and students will not be motivated to accomplish it.

Students are motivated when they understand why they need to do something, how it fits into the context of their world, exactly what you expect, and they want to give their input too.
Here is a real world activity I used in my classroom that helped students understand what boarding school life is like.
1) We were going to be reading A Separate Peace, John Knowles, and I wanted the students to understand the setting of the novel. I got online and contacted Phillips-Exeter (the school the story is based on, and where one of the film versions of the movie was shot), and I requested application materials.

2) On the day of the lesson, I simply stated to students that we were going to be learning about the importance of setting, and how our own experiences play a role in books we read. I had students brainstorm with a partner and write down their response to this hypothetical situation: "You come home from school and your parents ask you to have a seat in the living room with them. They calmly explain to you that they have enrolled you in a boarding school in New Hampshire, and that in one week, you will be flying out there to begin classes."

3) After a couple of minutes, I asked the groups to choose one of the partners to go and write a couple of their responses on the RIGHT side of the board.

4) Then, I put in the Phillips-Exeter admissions DVD and had students watch.

5) Now students worked with their partner to answer this hypothetical situation. "Your parents have just showed you this DVD, and asked your opinion about going to this school. What is your response?"

6) After a few minutes, I asked for the partner that did not go up before to go and write a few responses on the LEFT side of the board.

7) From here, I can take the lesson in many directions. I can have students look in the book and compare what the author describes to what they saw, I can have them write an essay on the setting, I can have them create an information organizer comparing public vs. Boarding school, or I can ask them for an opinion paragraph with well thought out supports for their opinion.

The point here is that I took something that seemingly had no value to students, the setting of a novel written 50 years ago, and made it matter to them, because I brought their world in (today's Phillip-Exeter on DVD), I asked their opinions (the board writing activities), I allowed them to work in real world situations (collaborating with co-workers), and I gave them a choice at the end of which assignment they would like to complete (we work out ahead of time rubrics for different types of assignments, so they know exactly what I expect, and they had a part in making those expectations as well.)

Make the lesson relevant, engaging, fast paced, and you will see students achieve and be motivated.