Your trip to Kerala sounds like a lot of fun. Since "being relaxed" can be done pretty much anywhere in Kerala, I'll pick the places you should go for the best "active" options.
1. Munnar - this has to be my no. 1 recommendation. A collection of vast green-tea estates, the landscape retains a classic hill station atmosphere. Watched over by Mount Anamudi, South India's highest peak (2,695m/8,840 ft.), Munnar's primary attractions are its gorgeous views of rolling hills covered with tea and cardamom plantations, and the cool climate. It's perfect for leisurely walks and cycle-tours (not to mention a chance of close encounter with the endangered Nilgiri tahr, a variety of mountain goat).
2. Periyar Tiger Reserve - There are a couple of lovely places to stay which the non-active members will enjoy, while you can opt for an overnight trek or a full day hike (note: avoid the crowded touristy boat rides). This makes for a good 1-2 night stop.
3. Serenity at Kanam Estate - this is an excellent place to remove yourself entirely from the beaten track and get a look at Kerala-style country life in a setting that really does feel far away from the tourist bandwagon. It is a converted 1920s bungalow in the heart of a rubber plantation, Kanam Estate. There are bikes for an independent tour of the nearby villages; guided plantation tours; bullock cart rides; cooking classes; and revitalizing Ayurvedic treatments.
4. Alappuzha - or the Backwaters, this is where you'll find great options for kayaking, as well as for the non-active folks to relax.
PS: since car is the best way to get around, I would strongly recommend against carrying a big suitcase each.
Hope this helps. I would love to help you plan the whole trip, including places to stay, choosing activity providers, etc. and plan a day-by-day itinerary.