I would begin by analyzing what individual sounds or events make up the composition of a meadow (and since not all meadows are created equal, the following categories are subject to change):
- Wind blowing through high tree canopies, tall grasses or bushes
Although this sound could be created using synthesizers, it may be more fun to create it using real props and microphones. To make wind sounds, you need a source of forced air and a surface to blow on or over; it could be your own breath, a quiet fan, or an inflated air mattress (shout out to @Utopia) blowing through some leaves from your yard. Experiment with different surface and air sources.
A little harder but still achievable. Most insects emit a scratchy, tonal sound, such as crickets, grasshoppers and cicadas. To mimic these creatures you may want to investigate small percussion instruments such as shakers, rainsticks, castanets and washboards. Create a large variety of tiny fragments of sound and then begin the layering process. With smart use of EQ, outdoor reverb and spatial panning this could turn out quite well.
Perhaps the toughest of all to fake! In the case of a meadow, many birds would be vocalizing melodic songs while others may be squabbling over food or territory (think crows!). You could try whistling a few melodies into your recorder and manipulating from there.
Best of luck! Sounds like an incredible project!