Tamiam you may not be able to read this being too late but I have to add other uses for ziplock bags. Put anything that may leak into a ziplock bag before you go. I had one tiny bottle of moisturizer leak all over my suitcase once and it was quite a disaster because the cloth bag I had the moisturizer in faded onto my one nice outfit!
To avoid having to carry bottles of shampoo and conditioner, (and I object to paying premium prices for the little travel versions) I just pour some into ziplock bags, but I them put them inside other ziplocks, just in case of leaks. Be sure to utilize the space inside of your shoes, you'll be surprised how many things you can shove inside a pair of sneakers as long as they are inside ziplock bags.
This may sound odd, but I also put all of my underwear in ziplock bags so that when the airport inspectors go through your luggage piece by piece (which they do often in some countries) it is not too mortifying. Ditto your dirty clothes in large ziplocks.
If you have anything that doesn't dry overnight when you are traveling, a ziplock bag is great to keep damp items separate when crammed into your suitcase. I like to keep a wet washcloth in a ziplock bag in place of the wet ones others mentioned.
On the issue of water, be sure to use the bottled water even to brush your teeth and to rinse your toothbrush.
On street food, it is probably fine if you can watch it being prepared. Anything that has been sitting out and could have been touched by flies is not a good idea.
Finally, on the malaria meds, it is only necessary if you are going to be in malaria areas. You can check the CDC website for information on the specifics for where you will be traveling. There is no risk in Nairobi for example. See
http://www.cdc.gov/travel. Your Masai walking tour sounds like it would be required. There are malaria meds available very cheaply in country and we have had students use them safely for years but new research in Europe has brought up concerns about their efficacy. To be honest, I wonder which drug company sponsored the research.
Check with the staff in your study abroad office at your college or university, they are a helpful resource on these topics.
Have a great trip.