I came across few interesting activities on the websites. One is called number line secret code and one is called lego secret code. Both activities will be great fun for students in the early years.

Lego secret code (Literacy)
https://frugalfun4boys.com/lego-secret-codes/
Students design their alphabet codes using a variety of legos. Build a sentence using legos and get a classmate to decode the data.

Number line secret code (Numeracy)

Number Line Secret Code Math Activity

Students draw a number line and label each number with an Alphabet. Students will then solve a seris of subtraction and addition questions, and use the number solved to find out the matching number from the number line.