As a teacher, I've always found the following items best for my welcome bags, in that they are VERY useful: nice pens (esp. red ones), plain ol' Post-Its, a few colorful postage stamps, a new chalkboard eraser, overhead type pens (They are a special brand, NOT sharpies), a few pencils. Post-it also makes page marker tabs (they are used like temporary book marks, and come in bright colors), mini highlighters, magnetic metal clips
Teacher will do everything they can to hold onto a nice pen - I'm not talking Cross - just a nice rollerball or felt tip!
One conference I went to gave out digital timers that could be displayed on an overhead projector, so kids could see the time left for an activity. These were VERY cool, and they couldn't have been very expensive. They were just cheap digital timers. I wish I knew where to find them. Any teacher would die for one of these!
At another conference, we got a set of ten small chalkboards (about 8" x 11"). These are great for lots of activites!
Things that tend to get never used: mousepads, rubber balls, stress balls, and toys (the kids discover them one day, and they become a liability)
sticky notes that have pictures on them or are any shape other than square(except maybe the school logo), Sharpies (rarely used, can also be mistakenly used on an overhead - ugh! since they don't erase), food, candy, block erasers, blank layards (though soem teachers loved the school logo ones one year)
Things I never got but would have LOVED: a gift card for a teacher supply store!, Staples, etc. - Even $5 would be great!, Kleenex TISSUES, hand sanitizer gel, unscented hand lotion
A company makes special staplers for teachers that fit over the hand. They are designed for using on bulletin boards. (They don't have to be held open like a regular stapler, so they can be used one handed )They are inexpensive and cool.
It also matters which grades you're talking about. Lower grades might LOVE those special scissors that cut a pattern (kind of like pinking shears), reward-type stickers or sticky stars, special magic markers (erasable)
I always wished for help getting posters too...since that ALWAYS has to come out of pocket, and you really can't have a classroom with bare walls all year. Those paper borders that go around boards are amazingly pricey too, but each teacher wants their own color/style. Maybe buy a bunch and let teachers pick out which style/color they want.