You can visit Taj Mahal in Agra from Delhi in one day and return back to Delhi easily. You can go from delhi  by Air, By Road by taxi & bus and also by Train.
There is an Airport in Agra and from Delhi to Agra, it will cost you a minimum of around NZ $235-$350 per ticket when you buy in India and the journey normally  has 2 stops at Gaya & Varanasi, with stopovers it may take around 24 hours to reach Agra.
 
However, you can hire an Air conditioned Taxi from Delhi in around $140 to $270 depending upon your selected car. It takes between 3 to 4 hours to reach there. There are several tour operators who conduct Delhi-Agra 1 day trip, they pick you up from your hotel in Delhi around 7 AM and you will be dropped back in your Delhi hotel in the night.
 
This website http://www.taxiguide.in/  will help you find all the details relating to distance, travel time and taxi charges for different cars for travel between almost all major tourist destinations in India.
 
There are also several Taxi Apps like uber which enable you to book taxi's on your phone.
 
MOST CONVENIENT - DELHI - AGRA - BY TRAIN in 1.40 hours.
The fastest morning train from Delhi to Agra is the new, non-stop 12050 Gatimaan Express. This train, which is India's first semi-high speed train, started operating on April 5, 2016. It travels at a top speed of 160 kilometers per hour, and takes one hour and 40 minutes to reach Agra. The train departs Delhi's Hazrat Nizamuddin Railway Station at 8.10 a.m. and arrives in Agra at 9.50 a.m. The fare in 1AC executive class (first class, seated, air conditioned) is around $32 per pessanger. CC (chair car, seated, air conditioned) is around $16. The train runs daily except Friday (when the Taj Mahal is closed). There is on-board catering, with both Indian and Continental cuisine served. The train also has hostesses and multi-media facilities.
 
Best Late Afternoon Agra to Delhi Trains
These Agra Delhi all have chairs, not sleepers.
The 12049 Gatimaan Express departs from Agra at 5.50 p.m., daily except Friday. It arrives at H Nizamuddin Railway Station in Delhi at 7.30 p.m. It's the fastest train from Agra to Delhi.
The 12279 Taj Express Superfast returns to Delhi from Agra at 6.55 p.m. daily (although it consistently reaches Agra about 25 minutes late, as it comes from Jhansi). It arrives at H Nizamuddin Railway Station at 10.05 p.m. 
The 12001 Bhopal New Delhi Shatabdi Express departs Agra at 9,15 p.m. and arrives at New Delhi Railway Station at 11.30 p.m. daily. However, the train consistently runs about 20 minutes late, as it originates from Bhopal. 
You can buy train tickets online also from Indian Railways website.
 
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