Umm See its all On you that how 15 days you want spend like visit whole kerala in 15 that is going to be speedy and hectic as well and a bit of rushy ..either way you can visit half of place but 2 days time at a place to have relaxed schedule. either way starting it with day one you will be getting cheaper stays at Kochi (cochin) reach there in morning get a hotel have rest and visit near by dutch palace and market . day 2 athirapally theres a good waterfall . and for kochi also day 3 leave for munnar you will love it .between the mountains and streams ....tea gardens and hotels between there is amazing you can have a trekk there in forest . there a museum to visit too . and day 4 there is a national park so spend your whole day there ... and also a lake by masonry dam and nilgiri tahr. day 5 leave for thekkady if you love wildlife or you can stay for long in munnar itself. day 6 periyar wilflife sanctuary . your night will be spent in periyar resort thats s lovely. if you wish to. day 7 next day for trivandrum ..kumarakon bird sanctuary ehere you will see a next level variety of birds . known place for orithologist paradise . then leave for allepy day 8 there s backwater in alapuza in alleppy live in a houseboat if you wish or just have a stroll in it. ejoy the dinner in it. and roam about thats a next level experience . leave for kollam day 10 again another houseboat city with an amazing lighthouse there to visit. you can then visit poovar . thats southern most tip to trivandrum nice place to stay . so here i am counting it 10 days or hardly 11 days you can also spend rest of 3 - 4 days as relaxation in between where you stay a day more if you like it.so that would be good and yes do search youtube for the want of new places. and how it looks .
Month of December is good time visit Kerala. Mostly people visit Cochin, idukki and alappuzha. I suggest you the same for the first few days. reach Cochin and from their you can travel to Athirappally waterfall and later to Munnar, thekkedi etc. returning back to Cochin you can explore the city and fort Cochin. Then go to alappuzha for houseboat experience and traditional Kerala foods. moving to the south situates varkala beach and cliffs, and at the southern tip situates Trivandrum which is also a good tourist spot. 15 days will be enough to visit these places. DM me if you wish to know more details. Always here to help.