April isn't really monsoon season yet in the north, so Rajasthan would definitely be recommended. We went last year in early July right before monsoon season started in earnest (it was late last year) and nearly melted in the heat. 2 degrees hotter and I'm sure my hair would have spontaneously combusted. But then again -- who goes to Jaisalmer in July?

The hill stations are very pleasant before the monsoons -- it gets a little rainy when the monsoons come. Shimla was great, as was Manali (the wild Mary Jane growing in the mountains was interesting...). Transportation around Himachal Pradesh can be iffy during the rains. Most of the towns are a rather significant bus ride, and the mountain roads can often be subjected to mudslides. The one freakiest experience we had in India was taking the night bus from Manali to Dharamsala. At one point, a landslide had covered part of a narrow mountain road, so we backed up and then CHARGED through the mud and uprooted trees. I think I found religion that night.