Both of those sites are totally doable in a day since they are right across the street from one another, I personally don't do it both because of cost (they are each $35-40 per diver) but it may be worth it for you to check it off your dive bucket list. If you're out doing one of those sites, Rainbow River is about a half hour south of Blue Grotto/Devil's Den and is a much cheaper drift dive ($5 park entry, $10 to take the boat upstream) - you'll want to be here before noon for the boat. For manatees, you'll probably have better luck snorkeling in Crystal River since they tend to avoid bubbles, but I believe you can also dive Blue Springs with them. They are protected so you can't initiate interaction with them unless they approach you first.
You should definitely do an ocean night dive! If you have trouble finding a shop or a boat to go out, let me know. I can get a group together that will help you meet your minimum, and know a couple boats who will do private charters upon request.