I tried your third example, and I got the same result you did. I could see from your grid that Puyallup could only be June 11 or 14, so I guessed Puyallup = June 11, and I got the following:
June 8 - 8:30 - Isanti - Zip Line
June 11 - 10:45 - Puyallup - Volcano
June 14 - 12 noon - Ephrata - Snorkeling
June 17 - 6:00 - Watonga - Horseback
June 20 - 2:30 - Barnhart - Rain Forest
Again, this seems to be consistent with the clues unless I missed something. I didn't look for another "solution."
These puzzles from the book have a different clue structure, so maybe there is some logic that I'm missing.