Sorry, quite late replying to this, but only say this section now.. ..anyway...
Echo what AntonG said. From a resources perspective, take a look at Tales from the Yawning Portal - plenty of smaller classic adventures there that have been converted to shiny new 5e D&D. The new Ghost of Saltmarsh book is also, by all accounts, a similar collection of shorter adventures that can be strung together into a single overriding campaign with only a little tinkering under the hood. Finally, from the official books, the second Waterdeep book (Dungeon of the Mad Mage) offers some quite neat dungeon levels that you can, with some work, adapt into a once-off adventure.
Another good resource is
www.dmsguild.com plenty of well made adventures and resources available for fairly nominal amounts (well, slightly less nominal after the recent moves in the ZAR).
If you prefer homebrew, then perhaps consider a set-up based (very loosely) on 'West Marches' type of campaign. Have a central city with an adventurers guild and a notice board. All players roll up their characters beforehand and then get to 'read' the notice board a few days before you play. Those who are able to commit to the upcoming session(s) then chose one of the offered adventures (basically the old quest-giver noticeboard trope from CRPGs). Then, based on their choice, you prep up said adventure and run through it. Then a quick write-up of the adventure can be circled around the wider player group afterwards to keep everyone involved. This can work well if you have a fairly large number of players with limited individual commitment- since only a few at a time need to be involved in each separate adventure. It also allows a fairly ad-hoc timing of game sessions. Finally, if someone loses interest or can't commit anymore - it's fairly easy to slot them out and slot in a new player....