Just hire a two wheeler and roam around Panjim, you will like everything in the small city of Old Goa. famous spots, churches, market areas are well marked round the corner. While returning back from Douna Poulo, enjoy your lunch at Mom's Kitchen with authentic recipes....
You should explore Meghalaya especially Cherapunji where you can find nature at its best along with several waterfalls. it can also be covered within 5 days.
North east sikkim Lachung and Lachen will be best for you. If you want to explore more of north east, you can have a look to this informative video I found on youtube https://youtu.be/iCNE-aI05PY
Kindly skip the cruise for Sunset .. it' will be better if you climb on the top of the Santorini ..there is a dedicated space where people gather for the sunset view ..
If you are traveling from Bangalore or Eastern Coastal Road, the roads are superb. You might be traveling faster than what your Google navigation will say. But, if you are travelling from Kochi/Western Coastal Road, it is a single road. Roads are good, but with a heavy traffic through out the day. What other locations you can visit depends totally on which route you're gonna follow.