One word: planning. If you're cruising to Juneau, you'll likely only have a day or two in port. If that is the case, prioritize your must-see excursions and book them well in advance. Once you've figured out the time blocks that those adventures will take up, fill the downtime with other, smaller activities. Take a walk through historic downtown Juneau if you have an hour between excursions. Or end your day relaxing at a local brewery.  Make sure you leave plenty of time to get from one destination to the other. And if the ship is leaving port, don't be late getting back!