Paradise is also a condition of happiness and peace. It's not just a place. Heaven is a place having a condition of paradise but so was the earth in the days of Adam and Eve before the fall and again it will be a paradise during the millennium of Christ's reign upon the earth.
The thief was specifically promised:
Luke 23:43
43 And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise.
But Jesus did not ascend into heaven on that day and he told Mary on the 3rd day that he had not yet ascended to his Father in Heaven. So obviously the paradise that the thief experienced was not in heaven but was with Jesus as he preached the gospel to the dead in the world of spirits.
1 Peter 3:18-20
18 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
19 By which also he went and preached unto the spirits in prison;
20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
1 Peter 4:6
6 For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
John 20:17
Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.
Paul in his teachings of the resurrection uses the ordinance of baptism for the dead to bolster his argument for the resurrection:
1 Corinthians 15:29
29 Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?