One of the best activities to develop number sense I've ever seen, even in upper elementary grades, is Contig! There is a Contig, Jr. for younger students.
I highly recommend any teacher to download (it's free here!) and copy a class set (one game board for every two students) and keep it close at hand for frequent practice. It makes a great center activity.
Follow this link to check it out. Please let me know how you like it!
Awesome Number Sense Game!