What they said above. Death Station/The Argon Gambit and Horde/The Chamax Plague are two of the best double adventures out there. The Traveller Adventure is a good campaign and it has plenty of extra cool stuff packed in it. Mission on Mithril/Across the Bright Face and Shadows/Annic Nova deserve an honorable mention. Research Station Gamma, Expedition to Zhodane, and Safari Ship were fun. Also, if you are into naval squadron/fleet action, then Trillion Credit Squadron may be right up your alley.