I'm looking for a number of Happiness items, specifically, those items that give the most happiness per square.
What would be the best things to buy to meet this, so I can keep buying shops, without using my entire map on happiness items?
Yeah, the new character pools, like Martha's pool, are the best (they're all the same except Tom's is a little less). You will still have to fill a good 3/4 of your map with happiness items (once you max out the map) to have some decent factory space xp You just need a crazy amount of happiness to do stuff.
Unless you really want better looks, I suggest picking one item type as your main so your game loads faster. And then fill in the odd smaller spaces with flowers or other happiness items worth more than flowers per square. I like to bunch all my factories together in one corner so you don't have to scroll around the map much.