Bordering Kenya and Tanzania, Rwanda's not a well known tourist destination. Few people embark on a trip to Rwanda. The country lacks infrastructure, so it's in your own interests that you pack well, as you'll have problems picking up supplies once you're there.
As a country that gets scorching hot, it's vital you pack enough lightweight clothes so that you can deal with the extreme heat. Once evening falls, you'll need long sleeved tops and trousers to protect you from blood thirsty mosquitoes. If you plan to go gorilla watching in the jungle, you'll need to protect yourself from the ever-present humidity. Finally, make sure you take a good pair of hiking boots.
As for medical supplies, pack the basics as well as medicines in case you get a tummy bug, plasters alongside an insect repellent.
If you plan to hire a vehicle and tour Rwanda at your own pace, you'll need to have an International Drivers Licence. You should also take a photocopy of your passport. You'll need it case you lose the original. Taking a debit/credit card is easier than travelling around with traveller's cheques. Finally, don't forget your camera/camcorder and enough memory sticks, so that you can immortalise your trip and show off your gorilla spotting photos to your friends and family.
Be prepared for a massive cultural shock once you reach Rwanda. So make sure you pack an open mind and spirit.