I got some static cling window tint in about 5% darkness and cut that to size for the windows I wanted blocked. It really cut down on light, didn't look tacky from inside or out and was super easily to remove.
http://www.walmart.com/ip/Auto-Expressions-Insta-Cling-Window-Tint-Extra-Dark-5/16672493Once we had our daughter, we needed a way to completely black out the windows during afternoon nap time. We bought Reflectix and cut to slightly oversized for the windows and bought some Velcro buttons (pieces of Velcro about the size of a quarter) and attached the Reflectix via Velcro. The Reflectix completely eliminates all the light intrusion, which certainly helps with summer heat gain. With the dark tint on the outside windows, you don't even see the Reflectix from outside.
Good luck! All told we spent about $25 and are super satisfied.