Hi Shani,
Sounds like a great time for you and your boyfriend! I am the official bag lady for our family and am always trying out the best bag options for our trips. My favorite bag is a small backpack option that allows me to keep my hands free. I love my Baggillini bag and you can find some great choices of lightweight bags
here. They offer a number of choices in different sizes with various pockets and compartments.
I like to carry a small ziploc bag with some first aid supplies, like antiseptic wipes, band-aids, Tylenol, antacid pills and hand sanitizer. Be sure to pack plenty of sunscreen and if you are not wearing a hat and sunglasses, bring some with you in your bag. I also make sure my bag has extra batteries and memory cards for my camera or camcorder. If you are traveling during the rainy season remember your rain ponchos and it is a great idea to bring a light sweatshirt as some of the indoor buildings tend to be quite cool. Some guests like to bring an extra pair of socks and change during the day to give their feet a break.
Most attractions are slow moving and there is plenty of space at your feet or the bench next to you to store a small bag. For the faster attractions like Dinosaur or Expedition Everest, you will find a net bag in front of your sheet to store your bag. I have never had a problem finding a place to put my bag during the ride.
Hope you both have a fantastic time!