Purchase a cheap set of magnetic letters. Let your child play with them on the fridge or on a cookie sheet.
Use your chalk to draw the alphabet on your sidewalk.
We love cookies and other sweets in our house and I am always looking for a good excuse to eat them, so I often use food for instructional purposes. You can purchase cookie cutters that are in the shape of letters, use frosting to write letters on your cookies, or purchase letter-shaped cookies at the store.
Letter Scavenger Hunt
Go on a walk or a drive and help your child look for letters. When we go to the mall, we sometimes look for our "letter of the day" on the signs in the store windows. If you have more than one child, you can make it a competition.
Purchase a few alphabet puzzles and help your child put them together. As he/she is working, point out the letters.
Read alphabet books
You can purchase books about the letters. Our favorites are Chicka, Chicka Boom Boom and Eating the Alphabet. You can also print alphabet books. Webbing into Literacy from Virginia has some very cute, free printable alphabet books (http://curry.virginia.edu/go/wil/home.html#Alphabet). You can also print alphabet coloring pages and make them into a book.
Alphabet coloring pages
Children love coloring and coloring the alphabet can be a fun way to play with the letters. Two of my favorite places to find alphabet coloring pages are http://www.first-school.ws/theme/bible/coloring-pages/alphabet-zn.htm andhttp://www.dltk-teach.com/alphabuddies/mabcposter.html
Letter Stamps and Stickers
My daughter loves art supplies and is always willing to work with letters if it means she can use her stamp pad or do stickers. Because finding letter stickers can sometimes be a little expensive, I often write the letters on cheap stickers that I buy.
For teaching letters, I love the website www.starfall.com. It has an interactive video for each letter and Abigail also loves the ABC song. Starfall also has an iphone/ipad app for the letter videos.
There is a plethora of free, printable alphabet activities on the internet that you can use. Here are some of my favorites:
Caterpillar - http://www.dltk-kids.com/crafts/insects/mcaterpillar-letters.htm
Flashcards - http://www.first-school.ws/theme/printables/flash-cards/alphabet.htm
Train - http://www.first-school.ws/theme/printables/alphabet-train.htm
There is an almost limitless way to incorporate letters into your day. Have fun and enjoy guiding and observing your child's learning.