Try finding a suggestion at K-5 Math Teaching Resources.
This website has loads of ideas to peruse through. You can filter your search by grade level, strand, Mental Math Concepts, Interactive Whiteboard ideas, or Centers.
The links to Interactive Whiteboard Resources take you directly to the IWB activity that relates to the concept.
There are many Mental Math Activities available, including a nice overview of using empty number lines in your classroom. And don't forget to check out the ideas for your 100th day of school.
This website has been a golden goto resource for k-5 math ideas and activities for several area teachers. Aren't we all looking for additional ideas?