After we've learned a math skill we're learning about (addition, subtraction, multiplication, and division), I have my students write math word problems and "mail" them to each other.
Step 1: Model several good word problems, so your students know how to write them, what to include, and what key words or phrases should be included.
Step 2: Have your students write five word problems on their own.
Step 3: Each kid is randomly assigned to "mail" the word problems to five different classmates.
Step 4: The classmate must solve all of the problems he/she receives and give them back to the original writer.
Step 5: The child who originally wrote the word problems is responsible for correcting their classmates' work.
It's much more fun than simply doing problems from a book, and it involves more critical thinking.