The covered trays from salad greens make great little greenhouses for starting seedlings. The empty containers that held mushrooms, hold 6 Jiffy Pellets just perfectly for starting my tomato or pepper plants. Cut up TP and paper towel rolls nicely line a standard sized flat for starting Peas and Sweet Pea flowers. I can fit 72 cells into a standard flat; then plant the seedling and pot when it's time to go outside. It's good for lots of things that don't like their roots disturbed. I might try beets or some colorful carrots that way this season.
I listed other things I'll be saving, below. Feel free to add your ideas, too!
Egg Shells - rinsed and dried, I crush them up and put them in the planting holes of tomatoes and peppers and summer squash to prevent Blossom End Rot.
Harvest Baskets - I used to be such a GARDEN SNOB and would ONLY collect my veggies in a trug or a hod, LOL! Now, when I am shopping a Thrift Store, if I see a sturdy (willow is best) handled basket, I grab it - they are useful and beautiful and when they finally fall apart, they can be composed or thrown in a fall bonfire.
Gallon Milk Jugs - Two reasons: They make great little 'cloches' for tomato and pepper plants (or anything else) but before I rinse them out for good, I fill them up once with the little drips of milk at the bottom still in there. I feed that water to my houseplants in the winter. I have Geraniums that bloom all winter thanks to that little calcium kick. Then, once that liquid is used up (I use a whole gallon to water my numerous houseplants each week) I rinse the jug out well, then string them on a piece of twine that's hanging in the basement for storage. I always need a jug to mix up some garden-related fertilizer concoction, too.
Coffee Grounds - Usually they just go into the compost, but I keep a small bucket of them under the sink to dry out and then sprinkle the dried grounds on the acid-loving plants that love them, come spring.
Newspapers - The B&W (non-shiny) sheets are great for making paper pots for seedlings, as well as shreds for the compost bin and a thick layer makes awesome weed barrier or mulch when covered up with shredded bark or soil. I've made MANY garden beds with thick layers of newspaper when I was without a tiller.
Cardboard - Since I order pet supplies on line (Chewy.com) I always have a stead stream or cardboard boxes coming to the house. FIRST< they get used as clubhouses for the cats, THEN I cut them down into big pieces, stack them, and keep them for using as mulch as you can with the newspaper layers, or to line the bottom of any new raised beds we're adding that season as a weed barrier. We are adding a minimum of TWO raised beds this season; four if I have my way.
Tin Food Cans with Cool Labels - I drill holes in the bottom and I just love how these look with herbs planted in them. Anything 'Italian' works for me. You can also use old tin/metal canisters or metal 'biscuit tins' that you find at Thrift stores, too. If you don't want to drill holes, make sure you have a layer of gravel in the bottom for drainage!
Large Styrofoam Meat Trays - same concept as the salad greens and mushroom containers, for seed starting with Jiffy Pellets.
Wine Bottles - Blues, Reds and Greens. I have a pretty 'Bottle Tree' that needs new bottles from time to time. Beau is going to build me another one with reinforcing rods that you usually use in concrete work. I've also seen fences that have wine bottles added to them - so pretty!