Don't be a boring teacher! Future card sharks love it when you pull out the math card games and they start having fun (who wants to grade worksheets anyway?).
What type of math activities for elementary school children should teachers be using?

The best ways to do all that? Games that use playing cards or math games using dice.
As Marilyn Burns says: "Games help to lift math off the textbook pages, and they support students' learning about numbers and operations."
They are also useful as centers, during free time, or during indoor recess. You will actually be surprised at how many students will come back to them long after your lesson is finished. 
The
other great thing about math games is that most of them are
accessible to all students. They also teach students to play both
cooperatively and competitively, and require them to use critical
thinking skills to win.
Math card games also require students to use their problem solving skills.
All of these games can be differentiated by changing a rule or two, using multiplication instead of addition, or by simply requiring students to prove their work to their partner through talking or writing.
Make 10
Remove all face cards from the deck. Deal 12 cards
face up. The players take turns finding different combinations that
add up to 10. When all possibilities are exhausted, more cards are
dealt for another total of 12 each.
Advanced students can make 10 with multiplication facts, using 3 addends, or division.
Memory
Building
visual memory and spatial reasoning is critical for teaching
mathematics. "Memory" is one of those math card games that does it
well.
Arrange all the cards (minus the face cards) face down
in rows. Players take turns turning over two cards at a time. If the
numbers match, the player winds the cards and takes another turn. If
they do not match, the are flipped back over and the next player goes.
Differentiation in the classroom can be done by using different variations of the game:
Addition Quick Draw
Deal all of the cards out to two players.
One player calls, "Draw" and both players turn over their top card.
The first player to add the two numbers showing and say the correct sum
winds the cards. The winner is the player who has the most cards.
Note: If an incorrect sum is said, that player must return a card already won to his or her pile.
Vary the game by using multiplication instead of addition.
Guess My Number
Place
a student at the front of the room and give him or her two playing
cards. The cards can be ordered any way the student wants. Be sure the
cards cannot be seen by the class!
Take turns around the room
guessing what the number is. The types of questions asked have to use
math vocabulary, such as, "Is it even?" "Is it odd?" "Is it greater
than 10?" "Is it a factor of 20?" and so on.
The student can only answer with a "Yes" or "No", until the actual number is guessed at.
This is one of the easiest math card games to differentiate!
Place Value Cards
For this game you need a place value mat up
to the thousands and their own deck of cards (remove face cards except
for Aces  these count as "1").
Students each have their own
deck of cards. The take the first five cards and lay them on the place
value math, placing a comma between the thousands and hundreds place.
The numbers must be said aloud.
The play could be for making
the largest number or the smallest number. You could also have them
write the numbers in expanded form and word form.
Fish Plus One
Remove
all face cards for early learning math card games. They can be added
in for upper level students, making the Jack = 11, Queen = 12, King =
13, and Ace = 14 or 1.
This game is played like "Go Fish," except that the card pairs have to differ by one.
Change the game up a bit by making it Fish Plus Two or Three. These are great math practice games for addition facts.
Toss Up
Each
player draws three cards from the deck and tosses them into the air.
Points are earned for every card that lands face up (Aces = 1, Jacks =
11, Queens = 12 and Kings = 13). The first player to reach a designated
score wins (50 or 100).
Differentiate the game by having to subtract any cards that land face down. You could also multiply cards and play to 500 or 1000.
Final Note: Games totally rock your students world, and you get to go home tonight and not grade any papers (but you do know they probably learned more doing this anyway, right?).