J. Eric Holmes' Portown is an evocative setting, with great potential as a home base to build a campaign around. There are several fan-made supplements and published modules that work well together, in this regard.
This classic illustration by Dave Trampier, from the AD&D 1e Players Handbook (1978) is what I think the inside of the Green Dragon Inn must look like, in Portown.
Back in 2010, I mocked up a map of Holmes' Portown, based upon the medieval city of Canea (modern-day Chania) on the island of Crete. I visited Chania in 2000, and was inspired by its historic old quarter.
Since then, others have expanded upon my roughly sketched map and key, such as RC Pinnell (aka Thorkhammer)'s Holmes Supplement: Portown (2013) and Piazza member religion's Port Gorod (2016) in the Grand Duchy of Karameikos, for the Mystara setting.
Most recently, Zach Howard provided a new map for Portown in The Ruined Tower of Zenopus and is working on The Forgotten Smugglers' Cave in the sea cliffs west of town. Zach's Portown Adventuring Eras is also worth a look, as a timeline for the setting.
Isle of the Abbey:
My introduction to the D&D game was the old, now legendary, blue book. From that introduction, I still retain a nostalgic fondness for low-level D&D adventures. The game has expanded immensely from those early, simple days, and my modules have expanded accordingly. For this adventure, I wanted to get back to basics (no pun intended) and do a module that required only the rules, monsters, and magical items from the Basic Set (either the old red boxed set of the new black box - the ancient blue book is not required).
Randy Maxwell, from Dungeon #34
The PCs are hired to explore the isle, in order to insure that it's safe for the mariners to claim. The adventure is designed for 4-6 characters of up to the 3rd level of experience (about 12 levels) with at least one cleric of 3rd level or two or more clerics of lower levels.
Of course, many of us prefer "the ancient blue book" and so I converted Maxwell's adventure for use with Holmes. You can download my version, here.
Corsairs of Tallibar:
"Corsairs of Tallibar" (1982) by Mike Wilson is a Judges Guild adventure for characters of 1st to 3rd levels of experience. The "Universal Fantasy Adventure" is system neutral and not associated with any particular setting.
The legendary corsairs were based out of a hidden island fortress, about 50 leagues south-by-southwest from the port city where the adventurers are located. They haven't been seen or heard from in 75 years, but their island has recently been located.
There are clear parallels between "Corsairs of Tallibar" and module B1 "In Search of the Unknown", which only add to its charm. A map of the island is provided, along with wandering monster tables for the different types of terrain.
I used the hidden island fortress as the secret base of operations for the pirates from the sample dungeon, members of the same group who attack the isle of the abbey, as opposed to a stronghold that's been deserted for 75 years, which worked out quite well.
Module B4: The Lost City:
Portown is described as "a small but busy city linking the caravan routes from the south to the merchant ships that dare the pirate-infested waters of the Northern Sea" and so I envisioned a desert to the south, the perfect spot for a lost civilization.
Module B4 "The Lost City" (1982) by Tom Moldvay included specific information for running the adventure only using the Holmes rulebook. This is an absolutely classic module, and one for which I pulled together a multi-author campaign sourcebook.
Moldvay's work is steeped in pulp fiction tropes, and meshes extremely well with Holmes. I can't think of a better follow-up adventure to the sample dungeon in the Basic Set rulebook, once the PCs are ready to leave Portown.