Thank you for your question. Several of the OT saints - mainly prophets were given glimpses of Heaven (eg. Daniel 7:9-14). However, the most notable and clear statement concerning a "mortal" gaining a "peak" at Heaven is in 2 Corinthians 12:1-4 where Paul indicates that he was "caught up to the third heaven." Of course, the Apostle John throughout the book of Revelation possessed visions that included a variety of scenes in heaven.
Hopefully this helps.