OP there were many vendors inJuneau. We booked a whale watching with MendelsonGlacier combo and had a memorable day.
We too left some things unbooked. On the top of our list was flight seeing over the glaciers which often gets grounded due to weather. Had we not been able to compete the flightseeing we prebooked during our land tour we wanted to try for it in Juneau, so did not prebook anything in that port.
fortunatly for us we did get to do flightseeing in Talketee, so also got a chance to see the magnificant whales by booking a last minute pier tour once we got to Jueau.
Best luck to you on your travels.