Traditional sembei are large, round and savory. Another type is arare (literallly "hailstones") - tiny and usually pellet-shaped, but also found shaped like animals (like the tanuki, or badger), seeds, maple leaves (momiji) or cherry blossoms (sakura).
A third type is kaki - bite-sized and sold in a variety of shapes. Sembei aren't all savory either; sweet sembei are made with wheat flour instead of rice flour.
One unusual variation is yasai sembei, vegetables that have been thinly sliced, covered in sugar and baked. Nuresembei are rice crackers that have been heavily doused with soy sauce and mirin, leaving a mochi mochi (chewy) consistency.
Look for genkotsu if you want something extremely hard, or try zarame (sugar-coated sembei) if you've got a sweet tooth. And a Tokyo shop called Mame Gen sells a popular snack made from nuts and beans covered with a sembei-like coating.
If you'd like to sample a bigger variety of sembei, head over to your closest department store food floor (depachika). You should be able to find it all there - savory and sweet, in various shapes and flavors. And different textures too: ranging from kari kari to saku saku to the unusual mochi mochi, you should be able to find just the right level of crunch for you.