The ones that are easy to put on generally don't make a ton of money. However, they're easy enough that you can do several different ones and the proceeds do add up.
Start by talking to restaurants in her hometown. Ask if they will do a fundraiser dinner. You work with the restaurant to set up a specific day and time, then create a flier to advertise it. Anyone who brings in that flier will have a certain amount of their bill donated to your cousin. If you have an area with several restaurants grouped together, you may be able to get them all to participate on one night, giving people a lot more choices of where to go. I don't know if you have any Chipotle franchises near you, but the ones in my neighborhood give 50% when we do school fundraisers there.
My friends recently made a few hundred dollars doing a lemonade stand to benefit a little boy with cancer. I think they sold cookies too. A lot of people who came to buy lemonade gave extra donations. If you have friends in several areas, you can all come together and do this (check out the Alex's Lemonade Stand site for some good ideas).
You can also do a group garage sale.
Set up a site through caringbridge.com or a similar organization. People can make donations to you through those sites. Be careful with caringbridge - if people don't use the proper donation link, the money goes to Caring Bridge itself and not to the designated recipient.
A Zumbathon is a good idea, but it will be a lot more work if you want it to be successful. You'll have to recruit a lot of dancers and come up with ways for them to solicit donations.
Whatever you do, make sure you advertise it like crazy. Do as much through social media as you can, pass out fliers at school or any organizations you belong to.
I'm so sorry for your cousin. I hope she's able to beat the cancer and that you can help with the expenses.