The worst things? Scented candles (allergies), things that were unsuited to me
The best things? Fabrics, books, chocolates.
I think $10 four times a year is an excellent idea. Hopefully you'll have people involved that really put some thought into what to get. The last time I had a Secret Sister it was through our church and the woman was the wife of a service man stationed in Afghanistan. Stationary, a journal, gas gift cards, Walmart gift cards, a book of stamps, a daily devotional book, etc. were some of the things I gave her. Since they had 3 small children I also tried to include just something small for each of them too. Sometimes it was just a piece of candy, very small toy or crayons but something. We didn't have a set limit.