Pens! In specific, fountain pens and dip pens.
I've always loved writing instruments of all kinds. Maybe it's because even early on, I've always felt that writing neatly and precisely in an organized fashion is important. I remember people saying that I obviously recopied my notes from class when I went home, which I never did - they just thought I did because my in-class notes were always neat and organized.
Part of the neatness and organization came from using the correct writing instruments. I used mechanical pencils in math class, even back in middle school, rather than a wooden one because the tip never dulled. This was back when mechanical pencils were expensive sturdy contraptions, rather than the inexpensive plastic ones you can get now. The fine point was perfect for equations and for chemical formulas. I kept that habit through college, as I majored in chemistry and took enough math to probably have qualified for a minor.
I do remember going to college and constantly losing my cheap plastic mechanical pencils, and having to go buy a new one almost every week. I finally decided to splurge on an expensive ($15! OMG!) heavy sturdy pencil, thinking that if it was expensive, I'd keep an eye on it. It worked. If I hadn't broken it years later, I would still be using it now. (I have a very nice one I use now, but the expensive heavy duty ones are difficult to find now.)
I don't know when I started to get picky about my pens, but I do remember in college buying this one type of felt pens with fine tips which were perfect for small writing (I write small) and for highlighting things with color. (One thing I don't use are highlighter pens. I've never really felt the need to mark up textbook passages with color.) Then, over the years, I've bought pens of all kinds, and though I don't use them much, they make me happy. Recently, while going through my room and clearing it out, I threw all the pens I found into a box - including sets of pens - and then afterwards, tested them all, throwing out the dead ones and giving the boring ones to my husband. There were easily a thousand pens there, and now they're reduced quite a bit, all organized, and I'm starting to use them in art projects. ("Art" is a relative term, by the way. For me, "art" is the same as "ruining a perfectly good piece of paper".)
Fountain pens, however, are my weakness. I use them at work and at home, almost exclusively unless I need a different type of pen for something specific. There are a lot of good inexpensive fountain pens, by Pilot, Ohto, Lamy, Platinum, and some others - you don't need to spend $250, or even $50, to get a good pen that writes smoothly and nicely. You can get regular nibs, or if you're so inclined, calligraphic nibs to make even my handwriting look good. And then there are tons of fountain pen inks to vary the color and style of your writing. (Always use inks that are designed for fountain pens, because many other inks will gum up in the pen and ruin it.)
The thing is, a good fountain pen writes smoothly and easily. I take fountain pens with me when I travel and write in my travel journal every night, and I can write for an hour without my hand getting tired; if I try that with a regular ballpoint pen, my hand will get tired within five minutes, just from the friction on the paper.
The only downside to fountain pens is that they can leak, but I've never actually had them do so short of dropping them tip-down. Even on an airplane, I haven't had the air pressure cause a spillage problem (though I do pack them in a ziploc just in case).
The most fun thing about fountain pens, though, is the pretentiousness. People don't realize that fountain pens are really just pens, so you pull one out and they look at you like you just stepped out of the 1880s. Really, though, the technology has advanced quite a bit and fountain pens work just as well as, or even better than, regular pens.
Want to try out a fountain pen? I recommend checking out jetpens.com for good, inexpensive beginner pens. I recommend the Platinum Preppy (fine or medium nib; extra-fine is hard to work with) as an inexpensive one you can carry around anywhere, or the Pilot Metropolitan for a $15 pen that looks like you spent $100 on it. Be sure to buy a couple of extra ink cartridges, and then when you really get into it, you can either get a cartridge converter to fill with specialty inks or a pipette to fill a used cartridge you washed out.
And then, if you want to see what's available on the higher end, you might try Goulet Pen Company for higher-end pens (none of which I've ever bought) and the coolest inks ever. What's really great is that they offer ink samples - little 2 ml. tubes of inks - so you can try them out before buying a big bottle.
Dip pens, I'm only just learning about. I can't really tell you much about them, and they certainly aren't portable, but they're fun.