In: Psychology
Since psychology is such a broad subject to explore, you can have n number of ideas to experiment on.
If you want your experiment to do something with a person's cognition, the stroop effect comes to mind which is a very interesting experiment, whereas if you want to explore the consumer psychology front, you can create your own experiment to see whether advertisements has any effect in consumer buying behavior or the relationship between salary and job satisfaction on the more industrial psychology front.
You can do a lot on the Learning side of things. You can explore problem based learning area. You can create an experiment on learning styles and how it affects problem solving mechanism (you can check out the nine dot problem for examples on syllogistic reasoning)
P. S. Check out the memory topic in psychology. One great experiment u can do on memory is Music and memory where you have few items on a cue card and the participant will try to memorize the words, first while listening to soothing music and then second, another set of cue cards with rock or hard music. Then compare the results and debrief.
Hope this helps.