Something personal for them, with a nice card is always lovely - the card is something they can keep forever and remember you by if you put in lovely message all the better. Anything such as wine, chocolate, flowers are very typical. For my Ancient history class my teacher collects a lot of spears, swords and helmets so we purchased at Saex sword and spear for her. My Modern teacher drinks a lot of coffee so my class made our own 'coffee basket of goodies' for her, which consists of different types of coffee, spoons, biscuits, and we ordered a fancy modern history cup online for her.
Really it depends on the individual teacher to what they would like. But most of all, they would appreciate the effort you went to purchase a gift for them, and they should love it anyway. n_n