Depending on where you are flying from 8 days or more is they way to go. First and last day are lost due to travel. You can spend the day at another resort in same town,but not the night. Sign up with romance rewards and when you get there you set it up. I think you need 24 hour notice.
In order to do a trading places day, you need to be a member of Romance Rewards. Once you are a member, you can visit the other resort on the same side of the island as your selected resort. Since you are staying at CTI, you would do a trading places day at CSS. Once you have signed up for Romance Rewards, and you've arrived at CTI, you need to sign up at least 24 hours in advance for your trading places day. Trading Places is available on Monday, Wednesday and Fridays I believe. You can visit CSS from about 10am to 4pm, and then a shuttle will take you back to CTI. If you want to go back to CTI earlier, that would be at your cost.