Dollar stores are your friend! Stores like Target or Wal Mart are great for the ideas I have, as well. With careful shopping, you can put together a really thoughtful gift.
Babies are so easy to shop for. They'll be happy with anything you get. Put together a bag of rattles, socks, teethers, baby shampoo and such. Maybe toss in a favorite snack or a cuddly animal.
I have 3 girls, ages 22, 21 and 10. At 9, all would have been thrilled with a gift bag of hair clips, bubble bath, child friendly nail polish and lip gloss, maybe all in a small purse. They also would have loved art supplies - paint and ceramics, or a jewelry making kit, or a small jewelry type box to decorate. It's pretty easy to find school supplies, jewelry or bath stuff in a character theme she'd love - say, Hello, Kitty, Hannah Montana or High School Musical.
My son is 14; at 11, he would have enjoyed Bionicles, Pokemon cards, Legos, used video games, books (the Warriors or Artemis Fowl series) or a DVD.
For adults, you could go with practical - a food basket with their favorite crackers and cheese; pancake mix and fancy toppings; kitchen gadgets (spatula, wisk, serving spoon); a basket of bath stuff or tools you always need but can't find, like a hammer, thumbtacks, and measuring tape. You could also go indulgent - paperback novels; candles; a bargain movie and microwave popcorn; chocolate and cocoa (my favorites!) or something related to a hobby - something to add to a collection (polished rocks, nutcrackers, elephant figurines- whatever!) Put together an emergency kit - first aid, a flashlight and batteries or glow sticks.