DX (dextromethophran) or CF, Guaifenesin (all found in cough syrup), chlorpheniramine (antihistamine) - If you take too much, these WILL depress your respiratory system and you can die. Better to take a small amount at a time, wait 30 min and see how it affects you. Mixes well with ginger ale!
Belladonna, Jimson Weed, etc - depress the CNS and cause severe hallucinations and death in toxic amounts. Here's a good site for toxic plants.
http://www.ansci.cornell.edu/plants/toxicagents/index.html
Beans - LoL, you can get "high" if thats the word, from raw uncooked beans. Kidney beans to be precise. Eating large amounts will make you hallucinate before becoming violently ill.
Zombie Breadfruit - not the regular breadfruit, but a fruit grown in the Caribbean that produces, yes, Zombie like effects. Natives take it all the time and its legal cuz no one really knows about it.
O2 depravation, CO2 inspiration - well, you will trip balls before you pass out or die.
Not sleeping - yes in 3 days of no sleep you will start seeing AND hearing things...Fun!
And finally this, which would be pretty trippy if I were actually shrooming or tripping:
http://www.youtube.com/watch?v=Vn1GaaLhz4g