To add a level of difficulty. I would add number cube to see how many letters to cover up at a time. Use tally marks, dots, numerals ect Free "find the letter" alphabet worksheets! Get them for uppercase, lowercase, and both!
123 Homeschool 4 Me has a fun, playful way to introduce alphabet letters to toddlers and preschoolers using kid favorite Lego Duplos. Just print these FREE Duplo Alphabet Mats and use them to teach your child letters and numbers in no time.