Here is a very elementary video worth showing your students in K-2
grade.  It is definitely very elementary, but Sesame Street does an
excellent job discussing subtraction.  It is quite entertaining.

Save the planet by destroying the aliens quickly.  Students need to solve addition problems in order to fire the weapon.  This will surely improve their additions skills and fluency.

In this game, students can compete against each other online to see who can get their jet ski to the end, the quickest by answering addition questions.

This is a fun song to help review or introduce basic addition and also math vocabulary to younger students.

In this 1st-grade math worksheet, students will determine which the whole number is missing from a number sentence. First-grade students will get an opportunity to demonstrate how whole numbers relate to each other in an equation. These numbers range from 1-20 and the math worksheet in a Valentine style, both font, and imagery. There are 8 number sentence questions.

Looking for a fun and engaging way to help your students review adding 1s, 10s, and hundreds? Look no further than Additionasaurus! This worksheet features a friendly dinosaur character that will keep your students motivated as they work through the problems. With a variety of addition problems that focus on ones, tens, and hundreds, your students will get plenty of practice and be well-prepared for more complex addition challenges.

This website gives a great example of using the commutative and associative properties of addition using Skittles.  There is a video available that will help demonstrate successful implementation.

Image anchor chart meant to demonstrate how to borrow when subtracting 2 to 3 digit numbers.

Multiplication.com has some really good self-correcting quizzes for math facts.  It allows students to take quizzes with visual hints or without them.  Students can practice math fact fluency for specific math facts that they might be struggling with.  When a student is finished it can even print out an image of how well they did.

Here are other links to 

3rd-grade students practice telling time using this worksheet.  There are 9 clocks where students have to either draw the clock face or write down the time that a clock is showing.  There is also 4 subtraction across zeros using 3 digit numbers as a review.