This is a fast and fun idea to use while teaching skip counting. Get a stack of multicolored sticky notes and a poster board. Use one color for the numbers that you are skipping therefor that you don't see, and use the other color for the numbers that you are actually saying out loud. We are skip counting by twos all the way till 20 so it all worked out nicely in two rows of 10 notes - 5 being invisible numbers and 5 visible.
I use this method for practicing skip counting after I already explained it to them and read them some books on that topic.
We used the same activity for skip counting by 5s and 10s. I had the kids switch the sticky notes themselves and they LOVED that! We didn't only say the numbers out loud, I also removed one of the numbers and they had to figure out which one was missing by repeating the skip counting in their head. We also stood up, faced the numbers, and read them out loud while jumping. For more advanced, you can turn around and say them without looking (watch for the mirror on the opposite wall, that I forgot about but the boys didn't :-D ). Have fun skip counting!