Mint leaves or catnip (cat mint)
It's relaxing, smells pretty good and is also good for stretching your weed, people have been smoking it for AGES.
Mallow, red clover and opium lettuce (wild lettuce) are also good if you go pick and dry them yourself. You ever wonder why bunnies get all sleepy after eating too much? X3
I read once that parsley helps your high but it's just too smokey everywhere -_-;
But for the love of god, don't just smoke anything random if you have no idea what happens when it burns. Erowid is a good starting point as aforementioned. Have fun.