I was in Delhi from 10th to 13th of April. Word of advice- do not go in summer as the temp can soar to 40-42 c.
The metro rail is amazing and you cannot go wrong if you follow the metro map which is available at customer care centre on every metro station.
Most of the tourist places are in the vicinity or else you can opt for hoho bus. Insist on autos with meter. The rate is 20 for first km and then 8. For traversing in the small lanes, try cycle rickshaws and battery operated rickshaws.
For foodies, visit to Chandni Chowk to taste parathas and jalebis is a must, for non vegetarians- chawla cream chicken is very good.
Shopping- If you are from Mumbai or any other megapolis, nothing spectacular! You have to haggle and bargain a lot with the shopkeepers with them try to cheat you at every step.
Hotels- Choose the ones which are near to metro station as travel time to tourist places reduces.