A night under the stars, just the two of you, is a great idea.
Let her think this is going to be just a quick trek on the trail, then surprise her by setting up camp. To explain why you're bringing the tent or why your pack is so heavy, tell her you're practicing getting used to the weight for a later, longer hike with friends. Pack everything you need to set the mood: a scented candle, her favorite nightie, your iPod and the speakers to play romantic music, a book of her favorite poetry to read to her and a light meal, such as crackers, cheese, some delectable chocolates and mini bottles of your favorite spirits. Take her on a trail with romantic spots to spend the night where you can watch a striking mountain sunset or the moon rise over a lake.
Indulge both his love of the outdoors and his love of food by surprising him with a picnic on the trail.
To explain why you're carrying a pack, tell him you put a weight in it to burn off more calories as you walk. Check the weather to make sure rain isn't in the forecast that day, then plan your picnic. You can keep it simple and pack a classic picnic meal of sandwiches, potato salad, lemonade and cookies, or you can go upscale and pack a fancy picnic meal with a bottle of wine and some hors d'oeuvres, such as ham-wrapped asparagus, stuffed cherry tomatoes and lemon bars. Walk the trail until you find a cozy clearing, a flat rock large enough for two or a place with a great view, then announce your plan.
Find romance in flora and fauna.
If she loves to see fall colors, hit the trails in an autumn month when the leaves on the trees are turning brilliant golds, greens, oranges and maroons. If vast green expanses are his thing, find trails to hike with breathtaking views of this kind of scenery. Another romantic idea is to schedule a hike when a variety of flowers are blooming. You can both simply stroll along the trail enjoying the colorful blooms or you can pick her a bouquet of wildflowers. Look online beforehand to find out the names and meanings of the flowers to tell her about them.
A fun project you can do together is pick fruit.
Certain farms, state parks and recreational areas let you hike their trails when fruit is in season, pick as much as you want and pay for your produce. Strawberries, huckleberries, blackberries, figs, raspberries and blueberries are just a few options. After you bring your bounty home, peruse fancy cookbooks or search online to find a recipe featuring the fruit that you can cook together. You could make a savory fig and goat cheese tart or some blueberry-glazed ribs for a romantic dinner by candlelight, raspberry pie, huckleberry-peach cobbler, blackberry turnovers, strawberry shortcake or a fruit salad for dessert.