As Jim said, either folders or collections (or collection sets) for grouping photos into "projects" - no hero image to represent.. sorry.
That said, it's not uncommon for people to have hundreds, even thousands of such "projects" - you just need to use the words (or click to see all photos) instead of relying on a representative image..
Consider a hierarchical organization with very descriptive titles (names) for your collections and sets.
Not sure what else to say, except give yourself a chance to get used to Lightroom. There will be a lot of things you miss at first, but less things as time goes on and you learn how to do things in Lr even if not quite as spiff as Aperture, or in some cases spiffier..
