Finally, back to the hex-mapping fun. What's the next step? That depends on how traditional you want to get. My game of choice these days is Labyrinth Lord/Moldvay/Cook B/X. The assumption of Moldvay and Cook was the players would graduate to hex crawling at the third or fourth level. Before then, the adventures would primarily be focused on dungeons.
If I was going to do things this way, I'd probably stop building the hex map now and refocus my efforts on one or two dungeon locations near the human city. It's still good to have mapped out the island as we have because I want to seed the dungeons with the promises of the hex-crawl. In this case, at least one of the dungeons would've been constructed by the wicked elves back when they ruled the island. I might also throw in a hint or two about the mind flayers or one of the dragons. I'd probably avoid using goblins since I want those to be one of the special parts of the jungle. I'd probably go with lesser undead, regular humans, and fantastical animals and follow the classic tropes of making each level more dangerous than the one above. I'd also strongly adhere to Moldvay's scheme of dungeon design in which a third of the treasure is in the hands of monsters, a third of the treasure is guarded by traps and a third of the treasure is undefended, though perhaps difficult to get to or find.
Optimally, the dungeons would be in, or very close, to the city. Part of the goal would be to solidify the human city as a home base for the PCs. One of the benefits of starting this way is that the players really learn their way around the human city. By this I don't mean geographically; the human city is mostly a safe place and I don't expect to do any urban crawling through it. I'm more talking about learning what resources the city has to offer and how to get them. They should learn which temples offer which services, what the alternatives to the temples are, if any, where they can (and can't) sleep safely for the night and store their treasure, and, of course, where they can acquire weapons, armor, and other supplies.
Finally, when I run hex-crawls, it can be important to have some idea of who the political and economic movers-and-shakers are in the civilized border area. There's an aspect of first contact in my hex crawls. The PCs are likely to find themselves ambassadors and go-betweens for both the human city and the monster civilizations they encounter. So there will at least be opportunities to explore the dungeons on behalf of, or under the patronage of, someone important in town.
I'm not going to go into too much detail here, because there are already some great resources for writing dungeons. With an eye on the above issues and prepping the campaign for the hex crawl to come, the dungeon should follow the usual design advice that works for low-level dungeoneering. That's not to say that they can't be unique. Only that there are well known methods for crafting a successful low-level dungeon and there's no reason not to use those here.
Next time, we go back to the hexes as we prepare to unleash our players upon them.