Summarize:

You can make quick cash by renting out your room for a short period, such as over the summer or Christmas vacations. Of course you will need another place to stay, and if you are renting the room yourself you will need agreement to sublet the room to someone else. For example, you may be able to rent your room to a visiting student who is only in town for a few weeks.  Advertise on notice boards around campus and online to draw attention to the room you are offering out. If you work away, and are always out of town for a few days of the week, you could consider renting your room on this basis. Some people will rent a room from Tuesday to Thursday, or regularly over the weekend. Sites that offer spares rooms or whole apartments to rent for short periods, to tourists amongst others, are becoming increasingly popular and prominent. If you need to make some quick cash, renting out space in your house can be a great way to do this. You could rent out a spare room, or, if you have somewhere to stay, you could even rent out the whole place.  AirBnB has become a huge business and gives you access to countless people online. If you do decide to rent out your apartment, be a little cautious and be sure you understand the terms, conditions, and liabilities you are undertaking. Try to vet your potential tenants as best you can.  The success of Uber means you can rent out your services driving people around too. In some big cities parking spaces are very precious commodities, and renting your parking space can earn you quick and ongoing cash. Your chances will depend entirely on where you are and what the situation is like there. If, for example, you live near a big commercial or business district where workers struggle to find parking you could be in luck.  Similarly, if you live near a transport hub, such as a train station, your parking space may be highly valuable. Make sure you are not violating any terms and conditions connected to your right to rent out your parking space, and then advertise it online. It's not exactly renting it out, but you can offer your body to science in exchange for quick cash. The possibilities and options near you will depend on where you live and what facilities are nearby. In the US you can sell blood plasma for around $35 for an hour or 90 minutes of your time, but you cannot in the UK.  Clinical trials take place in facilities and hospitals in cities across the world, and these can be very lucrative. There is an inherent risk with any kind of clinical trial so ensure you read all the literature you are provided with and think carefully about getting involved. You will be vetted and tested in order to ensure you are healthy enough to undergo the trial. You can find clinical trials near you by searching online.
Rent your room. Rent space in your house. Rent your parking space. Offer your body to science.