This is not so much Do It Yourself, as it is a Money Saver...I've been doing this for years, and it's been magic!
Problem? Crib/toddler bed mattresses are plastic. Easy. Pack n Play mattresses are not. Neither are twin mattresses for the older kids. Plastic mattress protectors are $10 and up in price (for a twin size...and I've never found a playpen sized one).
My solution? A shower curtain liner. Get the cheapest one you can! The dollar store has them for a dollar. I went high end on mine and got it at Walmart for $2:-) It doesn't need to be fancy or have magnets in it...it just needs to be plastic. It will cover a whole twin size with room to spare and tuck in etc (like a flat sheet), protecting the mattress from accidents etc. Fold it into approximate 4ths to cover the playpen mattress.
Then put the regular soft mattress cover over the plastic, and then the fitted sheet. Presto! If there is an accident in the night, the mattress is protected and the plastic just needs to get a wipe down with some lysol or vinegar (or whatever you use).
I even used these on my own bed (although the coverage is not quite as good on a queen size), when I was pregnant. You never know when your water will break, and I didn't want to have to invest in a new mattress:-) Also, they are a gem while traveling. I just bring a brand new one (its folded up so neat and small), and bring it along to whoevers house or hotel we are staying at. Slip it on the bed, and no worries about ruining someone elses things due to an accident or sickness.
There's my tip of the day!