Being a keralaite i can help you with that but do let me know your arrival city and time, mode of transport to explore the places (own vehicle, taxi, public transport)
We started from Ernakulam railway station to Munnar(2 days) to Thekkady(1 day) to Alappuzha(2 days) to Kochi(1 day) in the year 2016. We drove from one place to another in rental car.
Munnar
Thekkady
Allepy
Bekal
Kumarokom
are some options in kerala. You can club some nearby tamilnadu places also in a single trip. Include Kanyakumari and Trivandrum
7 Day is pretty good time explore Kerala.i was there last.month only. you can reach Kochi from your place as this place connected via railways,airways and roadways. Day1:
Spend your time in Kochi
Day2:
Leave for Munnar.Check-in to hotel.Do some site seeing.
Day3:
Go to top station and other sites.
Day4:
Leave for Thekady.Spend the night there.
Day 5:
Leave for allepey.spend night there
Day6:
Explore allepey ,houseboat and beautiful beaches
Day7:
head back to kochi.Explore spice market and jute market..Night or evening you can go back.