We've got two fabricated "cat trees" in our apartment, one which is almost to the ceiling in the corner kinda behind me and a smaller one that stands at the terrace doors.
The one behind me is right next to a deep bookcase we picked up at Ikea that goes along the wall next to my desk so our cat can hang out up there and watch me at my desk (often using the patch between me and keyboard to jump up and down from there actually).
The shorter one at the doors is just inside so that she can chill there and still look out, doors are open if it's not raining too hard (and we're home and alert). She is a "shade cat" - hates to be in direct sun - so she likes this one particularly in the middle of the day when it's sunny and hot out.
I like having a variety of perches for her with a variety of views, it helps keep her from being batty too often.
In addition to the perches, we've got a number of toys for her, some she can play with on her own and some to play with her. Even with these toys though, some of her favorite things to play with are used "stick" type things such as disposable chop sticks, straws and cotton swabs. If I don't keep her distracted when I'm throwing them away, she'll knock over the trash cans until she finds them, so usually I give her a fresh swab every few days.
As Kittster says also, cardboard can be a great "toy" too, if a bit on the messy side. If we have boxes due to get broken down, we usually let them be for a few days so Bonny can hop in and out and play games. We also have two "sheets" of cardboard that get tucked away and pulled back out every now and then, she loves to use them for scratching, just flat on the floor.