A few weeks ago I started a "Weekly Challenge" in my room. A little late in the year, sure, but oh well.
Each Monday, I post a new challenge, usually math related, and students have until Friday morning to take a guess. I make extra copies of worksheets so students can work at their desk (unless it's an estimation problem). I cut index cards into thirds, and that's where I have them write their name and answer. Once they have a guess, they put their answer in an old tissue box.
Some of the challenges I make myself, some I've found online, and some are just logic worksheets.
On Friday, I take out all the wrong answers and draw one winner from among the right answers. If it was estimation, I choose the closest answer. The winner gets something small as a prize, such as a mechanical pencil, eraser, homework pass, or small toy (play-dough, mini-slinky). When I did the goldfish estimation, the prize was the goldfish.
I put together some of my challenges into a PDF, as it's listed for free at Teachers Pay Teachers. You can download it by clicking here and going to the website.