Your not going to find much other than dextromethorphan, which you are already aware of, over the counter thats going to be much fun. Personally, there isn't anything over the counter that I would be interested in. I'm far past the days where allowance paid for my drugs and the circle of junior highers who can't find a source other than the swag marijuana. Unfortunatly I took it a little too far and 11 years since my first high, I've wound up in a methadone clinic, with an addiction to benzodiazepines which could lead to death if I went cold turkey. Injested everything on the table through a needle, straw, or pipe; reliant upon psychiatric medications. OK, I'm rambling.
However, I can make a list of some over the counter herbals and medications that may have some psychoactive effects, as minor as they may be, that you could use. I'm letting you know now, this list ain't going to be eye opening, after all, what do expect from legal substances? With the exception of our friend alcohol which causes more problems than any illegal drug...I won't get into that right now...
We all know that inhalants can get you high but I would never recommend such brain destroying shit, though I have nothing against nitrous oxide which can be fun, especially in combination with MDMA and a few other substances. Nitrous oxide isn't neurotoxic like those other inhalants like ether huffing, etc.
If your looking to keep stimulated, you can always check out a variety of over the counter stimulants. The more potent ones ( contain more than just caffeine and must be 18 years+ ) usually are found at gas stations and truck stops. I will list from least potent to the more potent...
Pseudoephedrine products
NoDoz or Vivarin or any other caffeine products
Stacker 2-3/ Contains caffeine+herbal stimulant formulas
Ephedra-Free Mini Thins and other caffeine+more potent herbal stimulants
Ephedrine HCL+Guaifenesin ( Bronchodilator/Expectorants )
If you are looking for relaxation, you can look around for Kava-kava which has similar but much weaker effects to diazepam (Valium). There are some preperations that work real well and some that don't. I'm not that intelligent in that field.
Some people get high by taking excessive doses of Dimenhydrinate which is Dramamine, an anti-histamine and anti-emetic. It is more a delerient than a hallucinogenic when abused. The high is frightening and lasts days, you may want to attempt suicide while on the drug. There is nothing fun about it, some people just always have to have their head twisted. Another anti-histamine which produces the same sort of effects when abused is diphenhydramine hydrochloride, the active ingredient in Benedryl. Again, why people want to abuse it? I dunno. Curiosity killed the cat. Once again, you do not hallucinate, you are delirious; this means you have no idea what is happening and your world is warped. Your lost. Your nothing. You don't understand anything. There is no such thing as a happy trip, every trip is labeled as a bad trip. Eat some fucking nutmeg, the trip is probably much cleaner, I can't comment, I was never desperate enough to try such things.
If you are lucky enough to live in one of the few countries where codeine is over the counter, you are lucky. This will provide the best high of any product over the counter, more euphoric than some prescription drugs.
This is depressing me. Get some real drugs. And once you get some real drugs, ask about some over the counter medications which may potentiate them