Upanishads say happiness depends upon three factors. One is the instrument with which you will enjoy the body. The environment also counts, it can't be this gross physical world. There's a whole range of heavens, not one heaven, multiple heavens. And by the way, none of them are particularly spiritual.
Swami Sarvapriyananda teaches the Katha Upanishad, which is one of India's most revered texts and a part of the Vedas. It is a collection of philosophical poems representing a conversation between the sage Naciketas and Yama (god of death). They discuss the nature of Atman, Brahman and Moksha(liberation).