I usually break up longer lessons/activities with a brain break. If you have access to a projector and speakers, GoNoodle is free and has tons of fun brain breaks. Also, Youtube has tons of songs and dances that go with numbers and colors. My class uses Dr. Jean (Lettercise is great!) and Jack Hartmann. StoryBots has quick, two-minute songs for each letter and number as well. I will usually play these a few times before asking my class to list things they saw that begin with the given letter. Sesame Street also has letter and number "podcasts" on Youtube, but these are closer to six minutes and can get to be too long.
For some non-video activities that my kids enjoy:
-ping pong letter toss: I put 26 plastic cups into one of those Sterilite plastic storage bins. Each cup has a letter taped to the bottom of the inside. Students take turns tossing a ping pong ball into the box and write down/name/trace the letter from their cup.
-scavenger hunts: (These are ridiculously easy to make and you can make them for anything!) Give students a clipboard if you have them or they can just write on the floor. The scavenger hunts my class does usually involve coloring in shapes, such as mittens, leaves, etc. that are numbered or lettered. Beforehand, I've taped the same shapes, but colored in, around the room. Students have to find the hidden shapes and color them with the correct color on their paper. These are great because they can work together to find things. I actually did one last week and had the kids hide the pictures around the room for each other to find.
-ABC yoga: This one I did have to buy, but I'm sure you could come up with or find a free version. The set that we use has at least one yoga pose to go with each letter, but there are some letters that have up to four poses. I like this set because there is also a coloring book available so I can make copies of the week's letter pose and send them home with my kids for them to do at home.
-Play-Doh and Lego letter building: My kids love these. First, they have to make the letter. I actually use plastic sleeves like these, but my mom ordered them for me off of Wish so I got them super cheap. Just pop in the new letter and you're good to go. After making the letter, they can build something that starts with that letter. I have pictures of things at the table to give them ideas.
-Pokey Pinning: This is the first year I've done this and I'm not sure how I feel about it yet. It's great fine motor practice, but some of my kids need several reminders that pins are for paper and not for their hands. Kids lay on a carpeted surface with a paper and a push pin. I found a set of free letter templates on Pinterest, but you could just as easily write the letters/numbers or have them do it for themselves. Using the push pin, they poke along the outline/line on their paper. When they're all done, they should be able to hold their paper up to the light or a window to see their image. I got these to hang their papers up on the windows, and they work okay (especially when the kids aren't constantly pulling them off of the window), but they're better than tape.
Those are just a few things that we use in my class, but I honestly go on Pinterest for most of my ideas. You can find a lot of good stuff on TPT for free, but if you're trying to avoid worksheets, that may be a bit harder to find.