Seed trays work as well as sand or salt trays. The smaller the seeds the better. Actually salt trays do not work in humid climates like in Rio de Janeiro, Brazil. Believe me I've tried that.
The red tiny seeds I used for my Seed Tray are called “ulezi” in Kiswahili. I have no idea what they are in English, neither in Brazilian Portuguese. Ulezi is sold in bulk in the local market. You buy the seeds, grind them and use the flour to make porridge, or to mix usually with maze porridge, as a local told me.
As with sand or salt we have to test the amount of seeds we put in the tray so you can see the bottom of the tray as you trace something. If the seeds and the tray have contrasting colors it’s even better. I chose a green tray, which gives a nice contrast with the color of the seeds.
The students are having fun with the new seed tray and it gives them a totally different texture than sand. It has been working great for tracing letters and numbers.
After you trace something, give it a little shake and the seeds cover the tray again.