My kids enjoy down time as I prep lessons with their alphatrain floor puzzle, I was lucky enough to find ours at a garage sale for next to nothing a couple of years ago. Not only does my toddler get letter and puzzle learning time, but he loves to crawl along the train when it's done shouting, "Choo-chooooo!" :) Even though my kindergartner knows her letters already, she still has fun with it, and because the letters are all scrambled in the box, it makes her think about the order they go in, as well.
Other activities I do to incorporate alphabet practice with a little indoor energy burning in the winter is I'll write the letters of the alphabet on pieces of construction paper and tape them down on the floor. Then the kids can jump from one letter to the next as they recite their letters, if they are different colors for the paper you can incorporate color practice for little ones, too. ("Jump to the Green B") I'll also use simple direction following with this, or Simon Says/Twister. ("Simon says touch the yellow R with your right hand," etc.) My kids love to do Alphabet Races where they run back and forth from A to Z and they burn up a ton of energy this way, it's a lot of fun! (Paper saving tip: cut the construction paper in half, you don't need a full sheet for each letter square.)
Another game we play during our circle time some mornings is: I have the alphabet on index cards and we'll put them in a bucket. It's kind of mixing the rules of mulberry bush and duck duck goose, but we sing a song, the kids pick a letter, say it and something that starts with that letter. To make it more challenging you can make it be something that starts with that letter that they can see in the room. What are some of your favorite letter games?