The MDO we used was a lot lighter on the curriculum, and fewer days/week, than some of my daughter's little friends, but we supplemented heavily at home (which I hope to continue doing throughout school), and we're well caught up, if not ahead in some things.
The Leapfrog videos are great fun. The Kumon learning books are good for little ones, too - mazes help with pencil control, and they have a great cutting book. I also like the publisher DK (Dorling Kindserly) - they have a series of books for younger kids - Eyewitness. They are still a bit mature for this age, but the pictures are fantastic, and we will save the more detailed caption descriptions for when DD gets older.
Words and letters MEAN something - labeling is an easy way to make the connect. Label easy things. (The fish tank has an index card that says, "Fish," a card on the door says, "Door," there is a picture of hands being washed on the bathroom wall, that says, "Wash your hands." Simple things like that.) Put the letters of the alphabet on the wall (we just taped a set of flashcards to the wall), and point to them while you sing the ABC's. We also got a dry erase board, and did the "Letter of the Day." I would draw the letter (we started with just uppercase but we're working on lowercase now, too) and we would draw all of the things DD could think of that started with that sound, and the second time through the alphabet, she would draw the letter, too. We are now up to sight words (DD was interested in learning to read, so I am definately taking advantage of that). We are doing a combination of sight words and phonics (Leapfrog does phonics, and it's working well with the sight words). I have a list of the words that our neighborhood kindergarten recommends students recognize by the time they start first grade, and we are occasionally learning a new one of those, when the mood strikes DD. (I figure that at the rate we're going, by the time kindergarten starts, she'll have enough, I hope, to make more structured learning less intimidating, but not so much that she's bored.) We write them on an index card, and then read a book together that has that word appearing a lot (it's fun to go to the library and find a good book with the right words, with the bonus that my kids are also learning how to be comfortable finding things in the library). After the book, the card is taped to the wall over her bed, and we review them from time to time, re-arranging them to make sentences. We not only work off of the school list, but also words that interest DD, or that we need to help our "playing" with the words - sometimes we need a new verb or noun to make things more fun.
Don't forget math - buy a piggy bank, and count the pennies as you drop them in. (Memorizing numbers and actually counting physical objects are two different skills.) Use your dry-erase board for learning what the numbers look like, too, and how to "draw" them. We also got a big bulletin-board style calendar at the teacher store, laminated the individual numbers, and bought sticky velcro from the craft store - we have the monthly task of taking down and rebuilding the calendar. It's into its second year now - that's why we laminated!
Opposites are important, too. "Go Dog Go" is not only a good beginning reader book, but it's riddled with opposites. And have fun with it - "Hello, Goodbye" by the Beatles is a favorite in our house.
The toy-like things from the teacher store are good, too: we have mini volcanoes and bug catchers (I drew the line at the 150-piece skeleton model - maybe in a year or two...). There are always fun, hands-on things. (I'm indulging every interest I can afford to/have the patience to - my goal is also to spark a general interest in learning.)
Don't forget that a lot of play is learning. While I'm clipping coupons on Sunday mornings, DD is cutting apart the sale ads. This is hand-eye coordination and learning a useful skill, as well as being fun for her.