I like to begin scatter plots with the height and arm span activity. You see different versions all over but it is a simple way to introduce this standard. And I can't figure out why but kids LOVE to measure each other and to be measured! I always have a few who have to measure their ears or nose or head! You've got to love 8th graders!
I found a great interactive notebook entry from the super amazing Everybody is a Genius blog. We wrote definitions and then matched the situations to the type of correlation it was. Great way to show real life and to get them thinking about the correlations. As they were measuring and recording, we made a spreadsheet of the class data. This helps them see the data form a correlation as everyone enters their measurements. Fun!