Hey there! The round trip Delhi - Paris will certainly be the best and cheapest option, as the total fare for the round trip will be $670 (while a one way ticket will cost up to $427 alone).
The best way to travel around Europe is by the most convenient and highly recommended Euro-rail. Some of the best sceneries around Europe can be seen from the trains. They are the most convenient and a very safe way of travelling to your desired destinations.
Here is what a traveller says:
Great way to see the countryside. Professionally run, ez to use, convenient, fast and no more than flying...We went from Barcelona to Carcassonne, then the next day to Dijon, then to Paris. Transfers were ez and schedules ez to understand... Trains well maintained. comfortable seats...watched the countryside while reading about our next destination and enjoying a grand burgundy! great way to travel!
(Source: TripAdvisor)
If you plan on travelling to and fro by the Euro-rail, I suggest buying the rail-pass. This will not only prove economical, but also has certain benefits like discounts on buses and ferry travels throughout Europe. Below is a snapshot of the pass rates. You can buy an online pass here.
The Thalys Train goes from Paris to Brussels. This train offers on-board internet access, free international pass, and also meal on board. There are a number to trains throughout the day, and you can book whichever is convenient for you.
Below is a snapshot of the ticket rates. Click here for full details.