sts7049 said:
you can take the eurostar to paris easily but that isn't a day trip. it's just over two hours of travel each way plus getting to/from stations, you will not have much of a day to explore anything. it's also pricey.
for bath there are trains that can get you there. the train trip should be a bit shorter, maybe just over an hour. i dont know if you can get to stonehenge solely by train.
I did the single day turn London to Paris and back twice for work meetings and once with my wife for her birthday. It's doable but it is a really long day. I'd probably do it over Bath for what it's worth. Bath is nice but you'll inevitably have a much more memorable day in Paris for a day than Bath. And both trips would be really long days. The Eurostar is more comfortable than the GWR too.
Being honest, I wasn't that blown away by Stonehenge.
If you do Paris in a day, you need to make sure you have a rock solid itinerary.