Use T1 Village of Hommlett
It has a starting level dungeon with 2 levels to it, one above ground and one below. There are monsters, traps, treasure, hidden secrets, paths, and doors, as well as potential captured allies. Plus, it has ties which can ultimately lead to a larger, actual megadungeon the Temple of Elemental Evil. However, that later one isn't quite as good, but you could build your own and pull in pieces you do like or toss it altogether.
Anyways, in T1 you have a fully fleshed out town too. That's important to old school play as you don't easily heal up in the dungeons. Towns are like allied dungeons - as long as you don't make the people there your enemies - and lawful ones like Hommlet have places to rest, heal, plan, and also rules for commerce, hiring aid, befriending others, as well as learning current rumors and history about the local area, people, and other inhabitants. It's as much a smorgasbord to explore and play with as any normal dungeon. And Hommlett has all of those things and more in it, if not as prominently marked as it could be.
The big key is: make this your own. It needs to make sense to you before you run it as well as agree with the Pathfinder ruleset. Which may mean finding a similar starting adventure, but Town and Dungeons aren't too hard to find.
Whatever you go with have an area map without too much beyond it, something you can "grow off of". Make your own material or purchase any of the Pathfinder adventures. They have some great designers there and a variety of styles for their adventures, not just adventure paths.