Mixed Model Regression in SPSS


I have a random group of participants that answered questions on a survey regarding their general opinions on specific health activities. Each participant answered questions about 4 randomly selected health activities (Swimming, brushing teeth, etc.).

My dependent variable is days per month spent doing a particular health-related activity.

I want to estimate the impact of opinions (questions asking opinions are the same for each health activity) on days per month engaging in the activity.

I already know that certain groups of activites are performed more frequently than others, simply due to their nature. I have grouped the 40 different health activities into about 6 different groups. 3 of them are below to give you an example:

Fitness: Running, Biking, Swimming, etc.
Hygiene: Brush Teeth, Shower, etc.
Diet: Eat Meat, Eat Vegetables, etc.

I want to know how to properly estimate the impact of opinions on the frequency with which a health activity is performed. I think this would be a simple fixed effects model with dummy variables representing each health group (6 groups total) if each particopant had answered questions about each health activity. But because participants were given 4 randomly selected health activities, I believe that the make-up of participants answering questions for each health group could skew the average dependent variable because some people will just be more health engaged than others. I want to control for the make-up of participants answering questions for each activity, which I believe is called random effects?

Can anyone advise? I am currently using SPSS for modeling. Thank you.