For a relaxed day with a couple or small group, I’d pick places where you can spend time together without rushing. Lalbagh and Cubbon Park are great for a walk, some greenery, and a quiet break.
If you enjoy food and cafés, Church Street and Indiranagar have plenty of options. Basavanagudi is also nice if you want local food, markets, and a more traditional side of Bangalore.
For something different, you can visit Bangalore Palace or spend a few hours at a local art or cultural space. I usually prefer planning two or three places in the same area, so there is less time spent in traffic.