Hi @kanchaysonqo -
You are going to have so much fun in South Africa! I spent several months in Cape Town and it is a magical place. I wouldn't miss out on hiking Lion's Head. It's a great route with some minor scrambling that has 360 views of the water, mountains and city. (If you're there when it's a fuller moon, it is popular to hike up for sunset and hike down to the lights of the city!) Table Mountain and Devil's Peak are other great day hikes.
Stellenbosch is the most popular wine region. It is lovely, but I also love Franschhoek! It is a little less trafficked since it is a bit farther, and is tucked up in some mountains and near a large Wildlife Preserve. You can't go wrong with either!
With a group of 15, I'd suggest checking out the Neighborhoods Market at Old Biscuit Mill if you have a free Saturday in Cape Town. Everyone can have their pick of what to eat for brunch/lunch and explore some local food and crafts. It's a great way to start the day before a hike or adventure around the city.
Kirstenbosch Gardens is a beautiful botanical space, and often has concert series. It is also worth renting a car or joining a tour that goes from Cape Town down through Fish Hoek to Cape Point, then loops around to visit the Penguins in Simon Town. Camps Bay & Clifton Beach are accessible beaches, and a bit further (or via train) is Kalk Bay & Muizenburg.
I feel like I could go on for too long! You're going to have a great time!