There are knit cacti everywhere and they are the most darling things I have ever seen. What's even better is that they are easy to make, especially for beginner knitters. This is the most brilliant way to use up odd balls of green wool you have at the bottom of your stash and the best part is, there's no real pattern to follow.
Use a smaller needer than the recommended size for your wool and just go for it, knitting a narrow rectangle. I've done mine in k1, p1 but I've seen others in moss stitch that look really cool too.
Once you have a rectangle as large as you want it, loop your tail through the remaining stitches on your needle to create the top of the cactus and stitch down the sides using a tapestry needle and the tail of your yarn. Stuff it and close, nestling it in a jar of garden stones or in it's own mini terracotta pot. Pin the hell out of it because these make mighty fine pin cushions.