Even though my experiment was only heating up liquids and measuring their temperatures, a margin of error still existed...
One of the errors I had was when I heated the liquids, I microwaved them. As a result, I couldn't watch their temperature increase (because temperature probes don't tend to do well in the microwave.) If I had heated them on the stove, I could have watched them heat and made sure they all heated to more or less the same temperature- I had to redo my water analysis twice because it started at temperatures way higher than those of salt water and oil.
Another thing I realized when doing my analysis was my method of calculating k didn't make much sense, because I kept changing the initial temperature. As a result, I entered all the data into a spreadsheet to analyze it, and found k for each data point. Then, I averaged these values. Luckily, my results were consistent and different for all the liquids.
If I were to change one thing about the experiment, I would test more liquids. How would, for example, a more viscous liquid like corn syrup or cornstarch and water cool? Also, I would like to test some semi-solid things (like jello or play-doh!) to see how that changed it.