Skills Students Struggle With

I work at an open enrollment college, which means a lot of our students severely struggle with math, or may not have had a math class in over 20 years. As part of my job here, I am running workshops on skills that they should ideally know, but greatly struggle with.

I am hoping you wonderful people can provide me with either some good short, interactive activities on various topics, or what topics you often see students struggle with. I currently have a session planned on adding and subtracting negatives using two-color tiles and one on adding fractions using fraction tiles. Any advice or resources is greatly appreciated!