As a resource room teacher, we do not have text books. We don't have a set curriculum (besides IEP Goals). I am constantly printing out worksheets that nobody uses because they needed help with a class project instead of the review day I've scheduled. Of course, when I am expecting the classroom teacher to send work, they don't and I'm left trying to figure out plans in the 30 seconds it takes us to walk to my room. I've found a solution. It's so simple, I can't believe I didn't think about it last year! Rather than printing worksheets and throwing them away - laminate them for an instant center/quick activity. You won't have to worry about running out of copies and trying to print out worksheets while students are waiting for you. When 1 student in a group finishes their work before the rest of the group (which will definately happen while your being observed), you will immediately have something for him/her to do. Being able to use wet erase markers rather than a pencil makes it feel more like a game to students.
My two favorite websites for these printable activities are themathworksheetsite.com and http://www.fcrr.org/
The math worksheet site is a math worksheet generator. If you join the website, you will have many more options/types of sheets however, there are a ton of sheets you can get for free too. FCRR is a website full of reading centers. They have them grouped into grade levels (K-1, 2-3, 4-5) and within each grade level they have printable activities for each of the 5 components of literacy (Phonemic Awareness, Phonics, Fluency, Vocabulary, and Comprehension). Most of the reading activities are card type games. I saved everything on my computer (in case they ask you to pay in the future) and I print off what I want onto card stock. There is a lot of work involved in making some of the reading games (cutting apart cards, laminating them, organizing the extra materials needed and setting it all up in 9x12 office envelopes - I'm also color coding the grade levels so I can tell the students what color games they can play.) I'll post pictures of some created games later.
Everything I'm printing from these websites I'm laminating. Now if my plans change, I need quick sub plans, or my students finish their work early - I'm prepared. It's also an easy way to build in review of previously taught skills. My favorite part is that once they are created, there is no more prep!
I hope this post has given you some ideas. Feel free to leave your favorite center/worksheet websites in the comments. Happy center making!