India is a very religious country, so the couple's religion can matter. If they are in different religions, do it in favor of whose side you are visiting for.
It is accepted if you leave cash in an envelope for the couple. Observe others to see whether they write the names in it or not, and follow. Usually, there are envelopes available on-site. I can't really comment about the amount though. Local currency should be OK.
If the couple if Muslim, there are some traditions they follow. I don't know much about them, but let's see if someone can comment on that. Money in an envelope doesn't seem to be a tradition.
For Hindu, Buddhist, Christian, etc, money should be fine.
There are print shops everywhere but it's a bit hard to find a good one. If you are planning to give them a printed gift, make sure you place the order at least 2 days before. Not sure if it's the same in the US, but I have seen some shops printing items in a very low quality.
A custom clock, canvas print of the couple, or something like that would be great in my opinion. I once gifted a couple a huge canvas print of them, and I still see it hanging in their place. TVs, rice cookers, and even cars are the norm there (Indian-made cars are very cheap).