I'm guessing you've researched nitrous oxide, but I'm compelled to remind you to watch out for vitamin B12 deficiency.
I have done this kind of thing in the past but try not to do so anymore. I noticed that a meta-mental pattern was forming, where I kind of started generally thinking in terms of rewards instead of just doing fulfilling things. And I didn't like that.
ETA: See overjustification effect.
I've read that Pavlovian conditioning can be used to trigger orgasms on demand, at least for women. Supposedly, you just whisper a particular word right before she's about to orgasm, and eventually, just saying the word outside of sex will be enough to trigger a less-intense orgasm by itself. Of course, the conditioning would wear off over time, as well as if it's repeatedly used without upkeep, but it could well be a fun and relatively harmless kinky thing for a couple to experiment with.
I'm a virgin, so I've obviously never tried this myself, so I suppose you might want to take this with a grain of salt, and of course, everyone's different so YMMV.
Certainly works for physical behaviours in my experience, (ie. making a particular type of touch associated with orgasm) don't know about words, but will ask some of the doms I know who're into that sort of control.
Funny... I've been thinking along the same lines (though I am not sure if this is technically Pavlovian). Except for nitrous oxide I was thinking of another highly pleasurable distraction.
I want to do some quick-and-dirty productivity hacks, along the lines of this or this. My simplified methodology is something like this: at the end of every 20 minutes of hard-ish labor (like writing Less Wrong posts or taking over African countries), I will flip a coin. If the coin lands tails, I inhale 8 grams of delicious nitrous oxide and keep on working. If heads, I die a little inside, take a 5 minute emailhackernewsfacebookblitzchess break, and then start working again.
The reason I really expect this to work is because I get significantly more pleasure out of nitrous oxide than I do from orgasms. It's that good. If positive conditioning works at all, this stuff had better do it. I'm skeptical that something else like a gummy worm would really motivate me. Plus I'm not too keen on ingesting excessively large amounts of sugar.
Have others done similar experiments? Any ideas for an improved methodology? Anyone else interested in trying this with their own drug/candy of choice so we can pool our findings? I'd be very happy if people used this to finally bang out that one Less Wrong post they've been meaning to write.