We FULLY AGREE with your river cruising philosophy. On our first river cruise we did both the morning and afternoon excursions each of the seven days. Although we truly enjoyed the excursions, we were exhausted by the end of our week aboard. On our second cruise we took a much more relaxed approach, and only went on the excursions that truly interested us. A few mornings we just wandered around on our own and loved that! We remained aboard the ship one morning when most guests took an excursion while the ship re-positioned downriver in their absence. We remained aboard and loved it. It was like having our own 445' yacht, complete with crew! LOL!!
On our first two cruises we immediately signed up for excursions as they became available at the 90-day-to-go point. On our upcoming cruise this May we're going to just go with the flow each day predicated upon what the cruise director suggests each evening during his/her briefing for the following day's events. If there are still seats/availability...fine. If not, fine as well. We can always take the ship's very nice bicycles out for a pleasant ride!