Silicone Stretch Lids
Link to buy in Canada: Amazon.ca
Silicone stretch lids can come in handy when you want an airtight seal over foods and containers you don't already have a lid for. There are many options for sizes (you can get all of one size if you find you use the smallest ones the most!).
I linked a package that has both round and rectangular covers that I believe this to be the most versatile. I own a 6 pack of rounds and love using it for half cut fruit (including a watermelon! In fact its the BEST invention for that!) as well as bowl covers, but the rounds don't strech over rectangular containers as well as I would like, which limits their versatility.
These really only work when you have a flat surface that needs covering (like a bowl top, or a flat edge of a half cut fruit), if you have less than half left, it doesn't seal airtight around the object and in those situations I prefer beeswax wraps. These silicone wraps however should last for many many years, so I believe a $15 investment will absolutely pay for itself.