I reason about the world using mental models. Simply put, a mental model is a tool for interpreting the world. 
As a college student, I make lots of decisions about how I spend my time. I want to have the highest returns on my decisions possible. I use my “asset allocation” mental model to help choose better time investments.
Asset allocation is an investment strategy that aims to balance risk and reward by apportioning a portfolio’s assets according to an individual’s goals, risk tolerance and investment horizon. 
Before deciding on what I should spend time on, I assess my options. I make time investments like how an investor would allocate assets.
I categorize all the potential time investments as asset classes. As each asset class has a different risk level, each potential time investment does as well.
There are plenty of activities in college that have fixed return. Most college courses, for example, have fixed returns. If I invest my time into Linear Algebra, then my return will be knowing how to do matrix transformations.
The risk in courses is little to none if you wouldn’t use the new knowledge till after graduation. In my case, most courses I can take during another quarter or self-study online. 
I recognize the importance of my fixed interest activities in maximizing value out of my life. But if I’m trying to have incredibly high returns on my investment of time, then I have to diversify and take risks.
Equities have more risk than fixed interest activities. You can have higher returns from an equity activity than a fixed interest activity, but you can also have lower returns.
Joining an extracurricular is an example of an equity. You could join an entrepreneurship organization and get to meet motivated people. But it could also be run poorly and waste your time, which could have been spent doing something else, like studying for a class.
Make sure that things you think are fixed interest aren’t actually equities! For example, a college course may seem like it’s fixed interest, but in reality, it can be an equity. If you meet great people in the class, then your investment had much more upside potential than a fixed interest activity. Then, choosing classes with great people is the real risk, which is higher than the risk in learning the wrong thing. 
One of my favorite personal investments that had immediate return was joining TreeHacks; I met a ton of cool people. I also had high returns from working with Matt Mochary, where I learned a ton about management and sales. Last but not least, I highly value building side projects with other awesome people, where I made great friends and became a better programmer. All these had high time investment risk, but in the end, they were tremendously impactful.
How I optimize for expected value
I aim to have a balanced portfolio that optimizes for expected value from my investments.
It’s important to have fixed interest activities to offset the risk from your equity activities. For example, I study for my classes in case my projects fall through. But it’s also important to have equities if you want to have potentially high returns.
Another thing I take into consideration is compound interest; it’s a type of interest that reinvests itself.  It plays a major role in choosing when you’ll invest your time into something. If an activity will completely change your perspective on life (in a positive way), you may want to invest in that activity rather than another activity that is fixed interest. That way, you’ll have compounding returns as you invest in new activities as a result of the new perspective.
Certain activities, like reading a book on how to learn better, have compounding returns. When you take a new course, it will be easier to learn because of your new knowledge. And you won’t have to use as much effort (capital) to invest and get returns, which means it’s a better time investment.
If you have a balanced portfolio, you’ll get better returns than the rest of the market (your peers) who may not have an asset allocation plan at all. And because time is limited and exponentially shrinking, it’s best to have a spending plan for it.
 If you want to learn more about mental models and the popular ones, read Poor Charlie’s Almanack or this piece by Gabriel Weinberg
 However, if you take a course that would change your perspective on a problem (i.e. in college), then not knowing the material in a course can be risky. It’s all in how you frame the time investment.