I've marveled at what can accomplish with one bag packing and the such out there, which has mostly been born out of necessity with rising airfare costs and fees. I shudder to think what it would've cost for my luggage when I returned from a semester in London had such rates existed then. Imagine a girl, proud of her new Anglicized vocabulary (you know, calling a cell phone a MO-bile), crawling through the plane aisles with a laptop case stuffed to the brim, a bookbag stuffed to the brim, and a huge plastic bags of gifts stuffed to the...yeah, you get the idea. This doesn't even include the two large checked suitcases that were stuffed solid.
Having said that, though, there are some things that I like to pack on every trip:
1. Compass/clock combination thingie that clips onto my coat. Since I generally don't bring my cell phone with me, I don't have a means of telling time so this way I can tell the time AND direction, which is useful when trying to find the next sight.
2. iPod Touch. Okay, I don't get into the whole Apple is Insert-the-Name-of-a-Deity here thing, but it's been very handy. I highly recommend bringing some sort of small mobile internet device (heck, I'll be honest: I don't even have any music on my iPod! Seriously, I must be some sort of freak) for travel. I love being able to check into my email quickly and for free and have found that in most of my European travels, the hotels offer Wifi (or, in Hungary, they at least directed me half a block away where I hung out on the steps of the university and checked on things at home). I especially like the Touch because it's small and fits in a purse/backpack/coatpocket/whathaveyou.
3. Lightweight, collapsible small backpack. I bought one from Eddie Bauer in a dark color (easier to hide the toils and stains of travel) that's lightweight nylon. I prefer it over a purse because I can generally stuff a sweater in it if I get too warm, a travel guide, water bottles and a wallet. I also buy a small travel lock to lock the zippers close. This is easily foiled by thieves but at least it's a small deterrent. It's a little bit harder to rip off than a purse is if it's on. Just be careful in crowded areas such as buses, where it'd be easy for pickpockets.
4. An inflatable neck pillow (shaped like a jellybean and I prefer the kind with the removable fabric covering for hygienic purposes). It makes that bus/plane/train ride a little more pleasant and it's lightweight and small and packs easily.
5. Depending on the weather, a coat that has an inner and outer shell. Since I generally travel on the off seasons (I LOVE fall and early spring travel) and because I'm a true Michigander all the way (we're good at staying warm), I couldn't imagine not taking a coat with me. I like to take a coat with a nylon shell and a zip-out liner. I've used one, the other, or both on trips. A bonus is that you can wad up the liner and use it as a pillow if need be. I highly recommend finding a coat with secure inner pockets. Keep your money and valuables on the inside. However, be aware that nylon type coats are rather casual and you're more likely to see European natives wearing more formal garb such as classic wool peacoats.
Stuff I'd like to buy:
I've heard that there are watches available for deaf people that vibrate for alarms. I think this would be very useful, especially in hostel travel, where one doesn't want to wake others. I need to investigate this further.
Women's dress boots that are stylish AND comfortable. I don't think that they really exist, or at least not for less than $100. Pros: dress boots go well with jeans or dress pants, help keep your feet and pants cuffs a little dryer in the event of rain, and aren't so casual as tennis shoes. Cons: heavy, take a lot of room when packing, and if they look good, almost always are uncomfortable. I'm always on a quest for the perfect pair of boots and have yet to find a pair that encompass all my needs.