A good activity to introduce or reinforce observations, inferences, and predictions is Animal Tracks (version 2)
I found a version of this lesson years ago and knew it could be adapted to make a great lesson on observation and inference for middle schoolers.
I start by briefly explaining the meaning of observation, inference, and prediction. Then I show the first third of the graphic on the PowerPoint. I give the kids some time to chat with their partner about what they observe. Then we follow up with a conversation about fact vs opinion. Some students make inferences at this time, so it's a good way to illustrate the difference between the two. Next we move on to inferences. Same procedure; team talk, then whole group conversation. At this point some students are making predictions, so this time to clarify the difference between an inference and a prediction. Then we move on to making predictions - team talk, the group discussion.
I show the next third of the graphic, do the same thing, then the final third.
The lesson takes less than one class period and is a wonderful springboard into doing what scientists do.
Liz LaRosa has another version of this lesson at middleschoolscience.