Honestly Little Mermaid is not a good choice for FP+. Ride that standby. I'd say at Big Thunder Mountain and Haunted Mansion. If you want to do Splash Mountain and Space Mountain, use rider swap. I'd say replace Jungle Cruise with Splash Mountain, and Ride Space Mountain at rope drop.
Here is basically my ranks for FP+ in Magic Kingdom
1. Seven Dwarfs Mine Train (Most Popular Ride)
2. Peter Pan's Flight (Also very popular)
3. Splash Mountain (Not great to do at rope drop because of getting wet).
4. Space Mountain (You can also do this at rope drop, but I kind of like to do tomorrowland later in the day, because most of the attractions are indoors incase of rough weather).
5. Haunted Mansion (Can have long waits and doesn't make sense to do at rope drop).
6. Buzz Lightyear's Space Ranger Spin (Same reason as Space Mountain)
7. Many Adventures of Winnie the Pooh (I usually do this at rope drop, and with 2 days it makes sense, however you can use FP+ as well)
8. Jungle Cruise (Thats a good ride to try for additional FP+ later, usually the wait time doesn't exceed 25 min).
Overall I usually do 2 days at MK as well, and I usually split it up like this:
Day 1: Fantasyland, Adventureland, and Haunted Mansion. I usually FP+ Mine Train, and Peter Pan for afternoon, and Haunted Mansion for late afternoon. I usually do Winnie the Pooh first thing, and then Belle, and Little Mermaid.
Day 2: Frontierland, and Tomorrowland. I usually FP+ Splash Mountain for right after lunch, and Buzz Lightyear, and Space Mountain for late afternoon. I like to start in Frontierland with BTMR at Rope Drop, then see Tom Sawyer Island, and then Lunch, Splash Mountain, and Country Bear Jamboree. Then usually go to Tomorrowland and work the other rides around my FP+ for Buzz Lightyear and Space Mountain.
But for you it depends if Splash and Space Mountain are attractions you would like to do.
