One of the simplest drills that can be performed anywhere can be used to increase computation speed and can be scaled for any skill level. The best part is that the investment is less than $5, and you are good to go for the rest of your time coaching. These tools rarely change and can be purchased online or in any shopping store in their toy aisle. What is the tool I am speaking of? Dice.
I started using the traditional 6-sided dice that I bought a pack of 6 for $2.99 at Walmart but then purchased a game dice set on Amazon for $5.00 to increase the difficulty for older athletes. I recommend a mix of 6-sided and multi-sided for your personal set, and you can even get large colorful ones for younger athletes.
I prefer to incorporate the drill during warm-ups, stressful training (post intervals), and during cool-down activities. The athlete comes up and rolls 2-4 dice and has a given amount of time to add, subtract, or multiply the numbers shown. You can also incorporate memory drills by having the athlete roll, take a visual snapshot, and walk away, being forced to recall the sum of their dice at the end of practice.
If you want to make this more of a challenging game, you can have two teams with their own sets of dice and run the drill as a relay where each team member has to run up, roll, compute, get the right answer, and return to the back of their line.
Multiple rolls with quick computations can be used for warm-up to get their brain turned on and ready to problem solve. This warm-up has worked great for team sports where observation and decision are a large part of the game (e.g., ice hockey, lacrosse, and baseball).
Another option is to include shapes on the dice sides and challenge the athletes to count the sides of each shape and add/subtract or to name a shape with that number of sides. Your imagination is the limit!
Give it a try, and let me know how you like it!