|I would like to add a fun quizzes to my website to keep visitors coming back - how can I make sure they won't get the same questions everytime they log in?
You can have more questions in your quiz than are asked at each attempt. Use the randomization to select a number of questions from a larger bank of questions in the quiz on each attempt.
If you're asking 10 questions on each try - set the quiz up to ask 10 questions randomly selected from the 100 or 1,000. Or have sections containing 10 questions each, and randomly select one from each section.
The default for generating pre-randomized quizzes is set at 5 - you can increase this to, eg, 50-100 to ensure a good random spread for 10 from 1000.