Is it possible if we don't book in advance but just knock on Jack's Diving Locker office to take a Manta Ray Night Dive ($105) on the same day?
in May around 12th - 14th.

coz we will start our trip from Hilo on 9th May, visiting the Volcano National Park, driving to Kona finally.
we just have 5-6 days staying on Big Island.

Any suggestions for me to spend the evening alone?
i cannot dive,
what can i do while my boyfriend is doing Manta Ray Night Dive?
Should we take Royal Kona Resort? so that when he is diving, i can swim in the resort safely? :P
(i am a poor swimmer)

I don't know anything about the night dive, but I don't think you would have a problem. Is there a phone number you can call? I have some Big Island ideas if you need suggestions.
As far as being alone, by all means, go to the luau! There is one at the Royal Kona Resort and my favorite, one at the King Kamehameha hotel. You won't be alone for long! You can walk there or get a ride from the hotel shuttle. It's SO much fun.
Yes- you can swim at the Royal Kona resort. We stayed there, the Kona Islander Inn and King K. hotel. Aside from the regular pool at Royal Kona, there is also a natural pool where the ocean water meets fresh water. You can swim with the huge turtles (don't touch them) and enjoy the sea life. It's a blast. The Royal Kona Resort is a very nice, very affordable place to stay. Feel free to take a look at my Big Island pages. I love that place.
Susan