Birdwatching tools: Many gardeners are interested in viewing and identifying the birds that visit their gardens. Birdhouses, bird feeders and birdbaths are great ways to invite birds into the garden; binoculars and bird identification books are great for watching and identifying the ones that visit.
Weather station: Gardeners are generally obsessed with weather, so a helpful tool to monitor a microclimate would be appreciated! The most useful units will track minimum and maximum temperatures and measure precipitation, while some will also keep track of barometric pressure, wind direction and humidity.
Magazine subscriptions: There are many gardening magazines out there, so try to pick one that is suitable to the recipient's climate and gardening style. Some magazines that focus on cold-climate gardening are Gardener for the Prairies and Gardens West. The Prairie Garden, an annual publication, also provides good reading. For more local ideas, browse the magazine rack at your local library or bookstore.
Something homemade: If you want to put your woodworking skills to use, nothing says "I love you" better than something you spent many hours creating. Grow light shelves, a potting bench, or a cold frame are all pretty simple projects that most gardeners would find useful.
Plants: Of course, if you're really uncertain, a gift card to the recipient's favourite nursery can always be used to buy new plants in the spring. All gardeners like plants!