Either give each student/pair/group a sheet with the alphabet on, or write the alphabet on the board. Students must come up with a topic related word beginning with each letter of the alphabet. A very easy concept that produces some interesting results with Key Stage 4 especially as they have to think creatively for the more awkward letters! For weaker students, fill in some of the letters for them or have these crossed out (typically Q,X,Y,Z), otherwise it can become demotivating.
For very competitive students (my Year 11!), you can set a time limit “How many letters can you fill in 3 minutes?”
It is great if you can leave the alphabet on display during the lesson so that words can be added during the course of the lesson or plenary.