My dd was a very early talker and seemed to have a very advanced understanding of reasoning and literature at an early age.
I have never really needed to know if she was intelligent or not: she enjoyed those things and they were fun so we did them.
Stimulating activities included talking to her, reading books to her, telling stories to her, listening to her when she told me stories, imaginative play, dressing up, drawing - just the same kind of thing that you'd do with any child, really. They have given her pleasure and stimulation that has lasted well into adulthood so that's good enough for me.
There is no reason why intelligence should make you grow bored with your toys: an intelligent child can create worlds out of wooden bricks or a pile of sand and some stones.
When dd said she was bored, what she actually meant was she wanted my company. She didn't need lots of specifically-designed-toys-for-the-highly-intelligent, she just needed me. And later on, friends to play with.
As children grow older, playing a musical instrument is a very good way of developing different areas of intelligence- and self-discipline too. Music is related to maths, as well as to the arts, and requires lateral thinking, particularly if you go into composing.