To experience the awesomeness of the scenery (the mountains are even grander when viewed in person), I'd go for the change of pace.
Weather will be your greatest variable, and frankly the biggest gamble. The trip reports for May this year showed a good number of sunny&70s days along with a fair share of gloomy,wet&50s days. And it wouldn't be unexpected to have both on the same day. You likely won't hit 40s except when close to glaciers or at night, though you can stay warm inside for those. Cold likely won't be your biggest issue, but a good chance of getting wet. Pack some good rain gear and you'll be fine. And know that the rain will be different than rain in Florida. Expect a steady drizzle more than a quick downpour.
Alaska cruises are perfect for walkers. In fact, on our next one we planning exactly what you're suggesting. Get the AllTrails app/website for lots of choices. You already have a good plan for Juneau. Through the app, we found some trails to try out in Skagway & Ketchikan. Both of which are also very fun, walkable towns with many cool shops, museums, and eat/drink establishments. In ISP you can do the short walk to Hoonah. It's a very scenic walk along the shore where we spotted whales and bald eagles our last time there.