We have use some of these for light days when we take a break from our regular curriculum. These printable reading comprehension exercises are for upper elementary through middle school. A change in pace every once in a while keeps things interesting and wakes up the brain.
Large selection from RHL school(includes contextual vocabulary)
Pearson Education. Online, but could be printable.
Also selected short stories from these vintage leveled readers. Most have reading comprehension questions at the end, or make up your own with Google Documents(free).
This is a helpful printable study strategy for increasing comprehension and retention. Good for middle school and up, but my eleven year old son uses this, so upper elementary can grasp it as well. Skim First, Answer Later: A Textbook Study Strategy.
Using the SRQ3 Method: Increasing Your Information Retention(printable)
Also, check your library with a "reading comprehension" search. I found a good number of children's resources. I always have to remind myself to check with the library because I forget that they do carry textbooks and other educational resources.