Paris to Strasbourg

TGV train is your best option and quickest.

Go to Gare de l'Est. Make sure you have a ticket before you get there. Board the TGV you reserved, and sit in the assigned seat. First or second stop will be Strasbourg. Pop off.

To come back it is just the same but the train will likely come into the station for you to board instead of being waiting for you like in Paris. Don't dawdle. Stand on the part of the platform with the sign for the car your seat is in. TGVs are long, don't wait for the last minute to go to the platform.

In between, enjoy Strasbourg, a lovely place. Don't miss the clock in the cathedral.

Thanks for the guidance. Train tickets seems very costly. Any other options (e.g., rent a car)?

Trains, especially "fast" or special service trains are priced exactly like airline tickets. You have to buy over a month in advance to get the lowest price offers, and you can't take a train at a different time than the one you actually booked. The only difference is that you usually can't buy as much as 360 days in advance, as you can with airline tickets.

https://www.ricksteves.com/travel-tips/transportation/trains

It is a sad truth that in many countries, long-distance day-trips like this border on cost-prohibitive if not planned (and rail tickets purchased) well in advance. For last-minute trips, most of us would probably shift gears and head to a place much closer to Paris that could be reached within a reasonable time by a regional train. Those ticket prices don't change as the travel date approaches.

Have you searched for tickets on the SNCF website? You might get lucky and find somewhat-better fares on trains quite early or quite late in the day. Don't use a third-party website like RailEurope; it might not be showing all the trains.