Well, personally I have a list of a few can't-miss places. Also keep in mind that flights within India can be relatively inexpensive which opens up your itinerary a bit more. Try
Clear Trip and
Ixigo for cheap flights.
Okay so if it was me and I had a month to kill in India before heading to Nepal, I'd probably spend more time around Rajasthan/Varanasi and the south since you'll be getting your fill of mountains in Nepal. I've been to India 3 times and will be going back for a 4th in October; if you want my opinion (
) this would be a decent sketch of an itinerary:
-land in Delhi, stay for a day or two. Don't miss the Jama Masjid and Qtub Minar if you get a chance.
-head to Agra on the early morning Shatabdi train to see the Taj, Agra Fort, etc. Maybe even Fatehpur Sikri if you have a little bit of time.
-Get the hell out of Agra before the day is over, preferably via overnight train.
-I'd probably head from there to Varanasi; spend a few days relaxing by the Ganges and taking in the sights and sounds (and smells). Something you will never, ever forget.
-After Varanasi, head to a couple of places in Rajasthan. I would really recommend that you pick just two or maybe three places instead of trying to do a whirlwind tour.
-I'd probably pick Udaipur and Jaisalmer to visit in Rajasthan but that's just me. Jaipur was okay, to me the main attraction there is Amber Fort, but the city wasn't nearly as beautiful or interesting as Udaipur especially.
-From there head via train to Mumbai for a few days
-From Mumbai head south, either to Goa if you want to be around other westerners and have a "party" vibe on the beach or Kerala for a little bit more of a relaxed pace.
-Kerala is really fabulous, Cochin is vibrant and interesting-- especially the older part of the city known as "Fort Cochin". From there you could try to do a night or two on the backwaters (from Alleppey). Definitely not to be missed if you can do it.
-My favorite beach town in Kerala is Varkala; in January/February the weather will be perfect and the vibe is very, very laid-back.
-Just south of Varkala is the town of Thiruvananthapuram (or Trivandrum). There's an airport there so you could fly back north.
Hope this helps.