Hot topic as a ton of more 'subtle' merchandise, honestly! You can get little headphone plug-ins for things like your cellphone or iPod, etc. Or they have iPod/iPhone cases more recently! Tons of necklaces (including a Spike one) and nice rubber bracelets (though they run at like, seven bucks a piece)
Not sure how full out you want to go on this, but Hot Topic has a sale on their hooded backpacks (see my notes on it), and they have a really nicely made Derpy one. It's very subtle and unless someone else watched the show, it's doubtful people would see it and go: Haha you watch My Little Pony. Here's a link to it: http://www.hottopic.com/hottopic/PopCulture/TV/MyLittlePony/My+Little+Pony+Derpy+Hooded+Backpack-631584.jsp (Hood can be hidden in a nice little pouch inside the backpack that is open/closed with a zipper- it's not obvious whatsoever)
Also, while the model is a woman, it doesn't look feminine by default. I bought mine recently and I think it is the absolute best thing I've ever bought, right next to my Fluttershy hoodie. The zippers have super cute muffin charms, and- just very nicely done, and subtle.
Bowties, if they're your thing? They also have socks! Or maybe you could get some pins or cutie mark patches to stick on things for some flair! There's plenty if you search around!