I currently have several cardboard boxes on windowsills in the porch in which I am growing seeds. There is no need to buy expensive plastic trays when you can grow seedlings of quick-maturing plants like mixed leaf salads and radishes in shallow trays cut from cardboard boxes.
Cardboard boxes are perfect for the job – they can withstand a fair bit of water without breaking down but then will be easily composted when they have done the job. Don’t throw out toilet roll tubes as they are also perfect to grow seeds in and can even be planted in the ground to decompose there as the plant grows. (This is perfect for starting plants which don’t like to have their roots disturbed, like sweetcorn.) I place groups of toilet rolls inside any food packaging trays we have saved, fill them with soil, plant away and then water when necessary. Easy.
While we are on the subject of recycling – plastic drinks bottles make perfect mini-cloches for early spring plantings. They could also save your young plants from mice and other rodents. When summer comes around, save the sticks of any ice lollies to use as plant labels. I also never throw away a glass jar – they will really come in handy later in the year when it comes (with any luck) to making jams and chutneys and other preserves. Fingers crossed!