You have a lot of options for excursions depending on which countries you visit.


Activity Level

The first thing to consider is how active you want your excursion to be. Do you want to be hiking around the hills of St. Lucia, kayaking with killer whales  or visiting the city markets and historical sites on a bus?  Many excursions will include warnings about the activity level. The excursions may also have age limits and weight limits depending on the activity. Excursions offered by cruise ships have activity levels. They vary by cruise line so a moderate excursion on one cruise line may be more strenuous than a moderate excursion on another cruise line.

Mode of transportation

The next peace of the puzzle is figuring out what you want the transportation to be. Do you want to be on a bus, a boat, riding bikes, in a private car, or even taking a Segway? This is a very important consideration on hilly islands in the Caribbean. If you get carsick, you'll want to consider taking a private transfer so you can ask the driver to stop if you start to feel sick.

Group Size
Group size is a big deal on excursions. Do you want to be on a bus with 40 other people? Would you prefer a private tour with just your family? Does it not matter because where you are going will be this crowded:
Cruise Excursion to the Vatican

 What's Included?

Are the meals included? Are the tickets into the Historic Site included? Does the excursion pick up from your hotel or do you need to meet at a central meeting point? These will all help you determined the best excursion for you. When traveling abroad, it's important to know if a tour guide of the sites is included. Tour guides usually have to have a special license so some tours include a "host" and a driver. You are on your own to explore the sites.

Customization
Is there a way you can customize the excursion? For example, we took a  group bus tour to Muir Woods from San Francisco. We purposely picked a tour that stopped in Sausalito on the way back because we wanted to get off the bus and take the ferry back.
View from Sausalito ferry

 That was our way of taking a generic group tour and customizing it to make it more of what we wanted.

 Cancellation Policies
It's critical to under the cancellation and payment policies. Are you making a deposit and the rest is due upon arrival? If you are on a cruise, what happens if you have to miss the port or the time in port gets changed?

Bonus Tip: When booking excursions for a cruise, be sure you understand the time zones. Some ships stay on "ship time" which may be different than local time. If you are booking a local excursion in local time, you need to make sure it lines up with the times you will be in port.