The first impression of Delhi, the capital of India, when you arrive is that it is a hectic city with almost 10 million people continuously moving, provoking constant traffic jams in the main roads. When you arrive there, you may be prone to get immersed into the city, and decide to walk to the closest spot from your hotel. We did that too, and those were the worst 2 km that I have walked in a long time. My first recommendation would be to avoid doing that: Delhi (and in general, India) is not thought to have people walking long distances. Instead, take its wonderful subway, you will be pleased with its modern network, and its tidiness.
If you happen to stay near the diplomatic area, your first stop is the Gate of India. It reminded me to the Puerta de Alcalá, but in a nicer promenade made to emphasise the presence of the Parliament. If you are there after the sunset, you will find a hectic environment: local tourists taking themselves selfies with the monument (hard with the light conditions), and some others offering all kind of services (pictures, water selling, sweets, …). The building might not impress you much, but the leisure atmosphere is a nice beginning.
From there, you can would head to Connaught Place, another particular spot of the city: well known brand (and expensive) shops, tons of restaurants and bars, bazaars in the street, and mostly young people make it the top destination for evenings out, and some decent -but expensive- shopping.
The real visit to Delhi starts with Old Delhi, and there the main attraction is the Red Fort: the first fort in India was also one of the biggest in the country, and if you started your trip now, you will enjoy this kind of palaces (after a few more, you may be looking for something else): a fort is a sort of palace with military defences. Forts are a good way to understand the aesthetic taste at different ages in the history of India, and you can imagine the rulers having their receptions or living there, walking like we can do now in their vast and well preserved gardens.
When you go out of the fort, crossing a busy bazaar, you will arrive the the Jama Masjid, an impressive mosque, able to host up to 25000 people. If like me, you find mosques captivating with their vast open spaces, you can enjoy its atmosphere of retreat, so needed even if it is only the first day in Delhi. You might be luckier than we were, and have a successful visit to the nearby bazaar and the spice market… We did not enjoy it much. Either we did not go to the right place, or my expectations were not the right ones. Bazaar and spice markets bring me to two places in this world: Istanbul and Marrakech, and that was not what we saw there.
To complete the day, you can visit two wonderful monuments; both of them are memorials of different rulers. The first one is the Safdarjang’s Tomb, and as it is not in the centre, it is not crowded, and its visit becomes a relaxing experience, where you can even take a short siesta to recover from the jetlag. The second one was the Humayan’s tomb, which is almost as crowded as the Fort. This memorial inspired the Taj Mahal in Agra, and it is a huge red marble building with several tombs inside. Taj Mahal is a clear must, but visiting Humayan’s tomb boosts the appetite to arrive to Agra.
Before the sunset, you can still be convinced by a crazy tuk tuk driver to go to the Lotus Temple, a modern building in the outskirts of Delhi, which hosts the Bahai House of Worship, aiming to close the gap between different religions… Not that interesting inside (a few minutes to reflect, pray, etc.), but really nice from the outside.
A great farewell from Delhi can be the Loddhi Park and buy some curry and tea. The park atmosphere was very similar to the one you can see in any big park in the world: my dear Retiro park in Madrid, Englischer Garten in München, Central Park in New York, with only minor differences (like the remains of a mosque inside): people jogging, couples, friends, and even BBQ’s… Life in megacities are the same in any place in the world: we are not that different in the end.