The cheapest option
Just use a duffel bag and a bungee net, but...
...if you have a little money you might consider an Eclipse Rumble Pack tail bag. It's 12x11x7" high (expandable to 12") and fits nicely on the rear portion of the 250 seat. It has an adjustable bungee cord attach system and the four hooks fit perfectly on the 250 bike hooks. Costs about $65.
Eclipse makes another nice one called The Fast Pack, and Tour Master, Ocelot and Lockhart make some good ones too.
Or you can make one yourself using a soft-sided cooler
It's an Eddie Bauer 18-can cooler from Target; regularly ~$30. Buy a $5 grommet kit at Wal-Mart (camping section) and install four grommets in the bottom four corners. Kit includes all you need; you basically punch a hole, insert top and bottom grommet pieces, and hammer them together; pretty easy. Then, use some small bungee cords (about 8 inches long and 1/4" in diameter) as follows: fold in half, stick loop through grommet (loop to the outside) and connect them in two pairs; one pair in front, one pair in rear. Hook them together with a short segment of coat hanger in the middle (if they're not quite long enough without it) and crimp so they won't come apart.
Payne has inspired some members to create a lockable, waterproof trunk for occasional runs to the grocery store and for storing helmet and cycling gear when hiking, picnicing, etc. Pretty simple. Cost a total of about $25.
It's a 56 liter Rubbermaid clone attached to the passenger seat, with bungees mounted inside the trunk and with U-bolts holding onto the grab bar for extra security. Wrap the U-bolts with electrical tape and put cloth patches on the trunk where it contacts the grab bar to keep either the U-bolts or trunk from scratching the grab bar.
This takes 2 minutes to put on or take off the bike. Attach the U-bolts, hook up the bungees, and away you go. It's got combination luggage locks that keep casual meth-heads out. A determined thief wouldn't have any problem just cutting into the side of the thing with a big knife.
On a test run to a distant grocery store the trunk stayed very secure, even in a downpour on the interstate. The paper grocery bags were completely dry. This thing doesn't have gaskets, so in case of a prolonged downpour, things might get a bit damp.
A rack on the back can make organizing a little easier. You might want to consider the Ventura rack.
Evolution of a touring bike
Here are some of the possible combinations of luggage for the 250. If you are creative and pack well, you can be gone for a good, long time on your little Ninja.
1. Bungee cords/nets are the only accessories used to hold this bag in place. Good for sleeping bag, clothes and such. Works well if not carrying many tools.
2. JC Whitney trunk mounted on the grab bar. A good setup for the money. Not much can fit into the trunk alone, for touring purposes, anyway. Middle and right pictures show mounting plate and location.
3. Here is a more serious setup. Still the same trunk, but used in a combination with the Ventura rack. The trunk is used to carry tools/tire repair kit/air pump/oil/chain lube etc. and some items that need to be easily accessed (such as rain gear). The cooler bungeed to the trunk is used for clothes. That touring setup is very good for long trips where you'd stay in motels (or a friend's house) and don't need any camping gear.
4. Same as 3 but with a sleeping bag attached.
5. And here is a serious touring setup. Includes full camping gear and full trunk of tools ... just in case. Rubbermaid Action Packer recommended; Cheap JC Whitney seat cushion not.
Most people don't end up needing the tools/oil/patch kits, (knock on wood), but having them on board may make you less worried.
6. An even more serious setup. 2-up camping trip. Good for a couple of days with a thin passenger, as the rear seat space is somewhat decreased. No rear grab bar, but the tent takes up a bit more space. The saddle bags are from Chase Harper.
7. Another serious camping setup. TourMaster Cortech saddlebags. A $10 compression sack from a sporting-goods store with 1" strapping sewn on by a local seamstress for $10 and 1" male buckles from ACE hardware ($1 each) on each end to mate with the female buckles on the saddlebags. Compression sack holds sleeping bag, pillow, and tent. Tent polls in small bag underneath straps. Sleeping pad attached to grab bar.