I have bought a lot of stuff over the years to make kids' learning easier and more fun: fraction strips, counters, dice, etc. Some of these get pulled out just a few times a year, others once a week, but there is one tool that gets used without fail every single day: the dry erase board.
Kids love to write on their own portable white board. There is less pressure to get everything right, or to make it look pretty the first time. Erasing is as easy as a finger-swipe, and there is plenty of room to solve and solve again.
Not to mention all the trees that are saved by getting rid of "scratch paper".
Here are just a few of the things my kids use a portable white board for:
I do recommend getting a magnetic white board. This way, your board does double duty. A lot of great math manipulatives are magnetic, and sight words are way more fun when kids can use magnet letters and write the words right next to them.
When buying a white board, check out the reviews first. The cheapest white boards often don't erase well and get marker stains that don't come off. Get a quality brand that will serve you well for many years. I recommend:
Dooley Boards Magnetic Dry Erase Board: It's magnetic, good quality, inexpensive, and at 12x18 inches, just the right size for kids.
Expo Low Odor Dry Erase Markers: Fine tip markers are better than broad tips for writing or solving math problems, and kids love the bright colors.
UPDATE: If you have an iPad, you can get a free app called "Paper by Fifty Three" that you draw or write on with your finger. Obviously it is not magnetic and the writing space is not quite as big, but it's another wipe-off option to consider for your kids.