Use the other hand. Use your non-dominant hand to brush your teeth, comb your hair or use the mouse. Write with both hands simultaneously. Just use it.
Block one or more senses. Eat blindfolded, wear earplugs, shower with your eyes closed.
Try to mentally estimate the passage of time.
Memorize people’s names.
Take a book and count the words in any paragraph. Count them again to be sure that it was correct. Once you conquer paragraphs, move on to counting words on a page. Perform the counting mentally and only with your eyes.
Sit in a chair and place a clock with a second hand on the table. Follow the second hand with your eyes as it goes around. Keep this up for five minutes, thinking of nothing else but the second hand, This is a very good exercise when you only have a few minutes to spare, if you are able to keep every other thought in the stream of consciousness subordinate to it. As there is little that is particularly interesting about the second hand, it is hard to do this, but in the extra effort of will power required to make it successful lies its value.