Half a day at Six Flags Kentucky Kingdom is worth it with your season pass. That might not even require half of a day if the lines are short enough. If you don't intend to go to Splashin' Safari [Holiday World's water park], then the rest of the day will be enough for Holiday World. Paramount King's Island is also close and has quite a few credits. However, if you choose Kings Island, you most likely won't have time for Beech Bend because it is in the opposite direction. If you do choose Kings Island, as long as lines aren't very long, it will not require a whole day. If you want to visit another smaller park in the area in addition to Kings Island, you can't find a list of nearby parks here: http://www.rcdb.com/locationresult.htm?nearpark=12&pstatus=141
Beech Bend will not take up the whole day and there aren't any other parks in the area. Have fun, the Voyage is amazing.
If you have any other questions, post them and they will be answered.