My cat looks bored out of her mind?

She is 7 months old Tortie Point Mitted Kitten and strictly a indoor. Has toys but could care less about them. I just purchased a 52 inch cat tree for her and its suppose to be here any day now. My older sister says its cruel to keep her inside the house all day. She should be allowed to out to explore. I had a harness for I used to take her out on but every time she saw my neighbor's outdoor cats she would get scared and run back to the house so I stopped taking her out on the leash. I don't want her out of the house for many reasons one especially because the cats and 2 dogs next door. She is also very pretty and people have asked for her but I said NO. I don't plan to get rid of her so I am afraid someone might steal her :/ I have this huge frame set up that I plan on to put her cat tree next to near the window. The frame makes the the birds chirping sounds and water fall sounds. She doesn't care about that either. She has been to the vet for a check up and is healthy. What can I do to keep her happy? Will she show any interest in the cat tree or is that gonna be a waste of money like all the other toys for her? She just lays in the sun and sleep, gets up and eats or uses the litter box and plays some and just plops down again like she is bored out of her mind.