One of the best ways to really learn about an area is with a local tour guide. A lot of your clients are going to want to utilize a service such as ToursbyLocals.com. They have over 4700 tour guides stretched over 190 countries. They can also drive around and pick up your client if they're so inclined.
The site also offers tours for shore excursions complete with a handy cruise search bar. Just key in the cruise your client is taking, and see a list of the tours offered.
ToursbyLocals.com is very easy to navigate and allows you to communicate directly with the guide you've chosen to get a custom price. The commissions here are low at 5%, so it will behoove you to check Viator too. Sometimes though, ease of use is worth the lower commission. Booking over $7500 worth of tours in a calendar year boosts you up to 10%.
BEWARE: There is no cancellation fee if you cancel 60 days or more out. If you're between 30-60 days, there's a 20% cancellation fee, and if it's less than 30 days out, the client will lose all their money. There is a CFAR (Cancel For Any Reason) option you can upgrade to, which will allow you to cancel with a full reimbursement up to 24 hours before.
Here's an explainer on how to use the site.
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