Alaska is one place I highly suggest booking the excursions on your own. Go through a reputable shore excursion company and often you'll not only save money and time, but it won't be anywhere near as crowded. We did several excursions and we were the only two on them!
You can actually do Mendenhall Glacier without an excursion. There's a bus that goes from downtown right to the visitor's center. The Glacier Express Blue Bus is $16 per person roundtrip.
As for doing everything you want to do, it really depends on the times. You'll want to give yourself some time in between just in case and allow enough time to make sure you're back on the ship in time.
One thing about Alaska, lots to see and do, and it's tough to do it all, which is why it's always good to do a cruise tour if you have the time and can afford it. Definitely an amazing place that everyone has to visit at least once in their life!