Can I safely integrate 3,3v sensor with 5v Arduino UNO? How?
If you need I2C the best solution is go for a logic level converter based on BSS138 mosfet or similar. I've used a few and they never let me down. Examples: Sparkfun have this https://www.sparkfun.com/products/12009 and Adafruit http://www.adafruit.com/products/757



if you have also a few transistors available, you can make your own homemade solution till you get something more robust, according to arduino page: https://playground.arduino.cc/Main/I2CBi-directionalLevelShifter


(source: arduino.cc)