Diphenhydramine HCl, the active ingredient in Benadryl and generic equivalents, is probably the easiest way to try an antihistamine trip. I recommend reading up on it on Erowid; basically, if you take enough, you'll see things, usually people and mundane objects, that seem completely realistic. It's not really euphoric, so most people don't find it fun, but if you're bored and need something interesting to try, it's an option. Take 300-400mg, make sure there are no other active ingredients.

Other things that are legal in some or all US states that are probably more fun/worthwhile than antihistamines include nitrous oxide, salvia divinorum and DXM.