I can't give you a detailed itinerary, but I can suggest some places that you must definitely not miss visiting,
1. Belur and Halebid
2. Hampi - Stay at Gopi Guest House
3. Charmadi Ghat - This can be a one day return drive
4. Udupi - Stay at the Welcome Hotel in Manipal
5. Pondicherry - Airbnb's, but make sure you stay in the French Colony, also visit Auroville when here
6. Alleppey - Houseboats, most definitely
7. Coonoor - Neemrana Hotel in Coonoor has to be the best bet
8. Kanyakumari
9. Rameshwaram
10. Trivandrum - Taj Kovalam, beautiful and elegant, stay here for a day at least.
For places to stay, you can look up Airbnb, they're the cheapest of the lot and the most convenient, that's what I've realised.
Whatever itinerary you choose, please try to cover ooty, coorg , allepey and old Cochin. I went on a week long bike trip from Bangalore last year, covering all these destinations. Loved fort Cochin so much that I ended up staying there 3 nights instead of 1 night as planned. In Ooty staying at a guest house with some local family is the best way of accommodation. In fort Cochin , try to get advance booking , as it's a heritage place now and stays have become pretty expensive .
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