Activities like boating, cycling, adventure activities and many more you can do with your partner in delhi.
Boating - Bhalswa lake, India gate
Cycling - Metro stations
You can visit this place if you're a bit sporty person or wanna try something challenging - Delhi Rock in GK 2. They provide many workshops like arm balancing workshop, there's rock climbing and much more
You can go to National science museum in pragati maidan it has alot of fun activites and science experiments which keeps you engaged as you explore the area. Plus you learn a thing or two so check it out of you haven't.