I found myself in this very predicament this week. Specifically, experiment 7.1 in Apologia Biology (involving extracting DNA from split peas) calls for meat tenderizer. Guess what I didn't have on hand because it's something I don't use? You got it - meat tenderizer. Rather than go out and spend money on something I won't use again, I subbed the experiment. I had my son extract DNA from a banana instead.
Straining the mixture
Waiting for the liquid to drip through (the glass is 'foggy' due to hot water being used)
Waiting for the DNA layer to clump between the banana liquid and rubbing alcohol
Stirring, stirring, stirring
DNA! You can't see it in the photo, but I promise it's there.
There are a variety of sites with instructions for this particular experiment online. I used the one from Scientific American. It's easy, doesn't require difficult or expensive supplies, and didn't require a lot of prep work or time.