Some of the best adventure/romance combinations are related to post-apocalyptic or dystopian novels. In the best of those, the scenarios are plausible, without fantasy. These are my favorites - kind of like The Hunger Games:
Shipbreaker by Paolo Bacigalupi - In a near future where oil has run dry, grounded oil tankers are scavenged for parts by teenage crews. Seventeen-year-old Nailer works the dangerous duty, hoping to survive his job and his murderous father. When he discovers a luxury yacht wrecked by a storm, he faces a difficult decision: scavenge the ship to become rich, or rescue the sole survivor, a beautiful, wealthy teenage girl. His decision propels him into more danger, adventure, and love than he had ever counted on!
The Shore of Monsters and Fall of Darkness by David J. Nix - Five generations earlier, a horde of monsters nearly obliterated humanity. All males are dead or ruined by a monster plague; words like 'father' and 'romance' have lost meaning. When teenager Sky joins an expedition to the shore that falls apart, she must survive amongst the monsters that roam the ruins. She gets unexpected help from a very surprising source. Mystery, action, and romance follow!
The Forest of Hands and Teeth by Carrie Ryan - Seven generations have passed since the Return, a plague that reanimates dead humans into creatures that feed on the living. Teenager Mary lives inside one of the last enclaves of uninfected, protected by a chain link fence that surrounds her village. When the fence is breached, Mary flees the village with a small band of survivors. Their flight toward an uncertain salvation is both harrowing and revealing, as they try to determine if they are humanity’s last hope.
How I Live Now by Meg Rosoff - Fascinating novel about the outbreak of a 21st century world war as seen through the eyes of Daisy, a 15 year old American staying with her cousins on a remote England farm. At first utopian, the kid's existence degenerates into horror as the war encroaches on the farm. Through the several month period covered by the story, Daisy grows from a self-centered girl into a determined survivor. This book will leave a mark on the reader for years to come.
Tomorrow When the War Began by John Marsden - In a fantastic coming-of-age novel, Ellie tells the story of the invasion of her homeland of Australia by a foreign power. She escapes to the outback with six of her friends, where they make plans to fight a guerilla war against the occupiers. Ellie tells the story with superb sensitivity and insight, which generates the power that makes a good story a remarkable one. The relationships amongst the teens are incredibly realistic, and the accounts of armed conflicts are riveting.
Uglies by Scott Westerfield - In a future society, a mandatory operation at age 16 wipes out physical differences, turning "Uglies" into "Pretties". The Pretties are allowed freedom to play, while the Uglies jealously await their turn. Ugly Tally has gotten into trouble that may forfeit her operation. The menacing government offers her a way out: find a group of rebel Uglies, infiltrate, and betray them. Tally agrees, but upon finding the rebels comes to understand the terrible price of becoming pretty.
This one has less adventure, but very romantic and mysterious: Jellicoe Road by Melina Marchetta - Taylor finds herself the leader of her boarding school's Underground Community, whose mission is to battle their counterparts at an all-male Cadet encampment and the public high school. For two decades the three factions have waged a game of negotiation, pranks, and sometimes physical violence for control of paths, places, and facilities. One problem: Taylor might have a crush on the leader of the Cadets. The author weaves in a back story about how the whole war started, and how Taylor is unwittingly tied to it. Romance, action, mystery, and plot twists, this is just a great story.