The Great Nazarene Teacher said that the kingdom of heaven is within you.
From my limited understanding, Heaven is a spiritual condition -- one of wholeness or togetherness with God. It is also spiritual wakefulness. When the consciousness of spirit (our true selves) is active, bodily death no longer is an end to consciousness. This is the everlasting life of which Christianity's founder spoke.
It also seems, from my own exegetical research, that the Garden of Eden was the Garden of Heaven -- purely and only a spiritual place. Such a "place" has no location in space and time, but is superior to all physicality.
Heaven is also a place of perfection where generosity is not the opposite of selfishness, where confidence is not the opposite of doubt, where wisdom is not the opposite of stupidity and compassion is not the opposite of indifference. All of these negative traits do not exist there.
Heaven is bliss grounded in the most perfect reality -- that of creation, where spirit, inspiration and forgiveness originate.