Well, my advice would be, make the romance deeply connected and relevant to the story.
You shouldn't fit the romance into the adventure, but instead deeply link it to the adventure, to the point you can't have one without the other.
Now, for their relationship, the first things you've gotta figure out are:
1 - Their dynamic, this will help you make their interactions entertaining and memorable, and considering they will be the heart of the story, you need to have a solid basis of what their dynamic is. One of the classic dynamics is the funny one and the serious one, for example. But you can basically do whatever you want, your options are basically infinite, I would recommend making them character foils though, which means, giving them contrasting characteristics that will clash with one another. Not all of their characteristics need to be contrasting though.
2 - What do they see in each other, basically why are they attracted to each other? Is it just because they are pretty or because the plot says so? Basically, just find the answer to the following question "What and how this relationship will add something to their lives that they hadn't before?".
3 - A problem, basically what stops them from just hooking up on the spot. Now, not every romance has this, and you obviously can write a story without a problem. But, if you want some kind of conflict in their relationship, you should probably think of some elements to get in their way. It can be either external elements that get in their way, such as their families hating each other, or internal elements, such as immaturity.
Once you have figured who they are, what they want, how their relationship will go, and what gets in their way, you're pretty much ready to go. The plot itself should just be events to lead to the story's emotional beats and the progression of their relationship, and maybe some flavor depending on the events and environments, but that is it.
Your focus should be the characters here.
Hope it helps.