Day trip to Assisi from Rome

Yes this is easy to do. That route is served by Regionale or regional trains.
You can search for the time you want to leave and come back on
https://www.trenitalia.com/

You have to use the Italian spelling for the stations in this case Roma Termini to Assisi.
Factor in that the last train from Assisi back to Rome Termini Station is at 20:51.

Also the train stop is about 2 miles from the town (but it is walkable to some I walked it along the old path that pilgrims use) and you have to either take a frequent bus (the tickets are sold at the cafe in the train station) or take a taxi to the town

Assisi is a lovely place, but it's pretty far for a day-trip. On weekdays there's a 7:58 AM train (I'm assuming you aren't interested in anything earlier than that); it reaches Assisi at 10:07. The next train (at 9:28 AM) is a little slower and doesn't arrive until 12:02 PM. As already mentioned, the Assisi train station isn't very close to the basilica, so you'll need to factor in some additional time for local transportation. And of course you need allow for the time it takes to travel between your hotel and Roma Termini (or Roma Tiburina if it's closer) at both ends of the trip.

Assisi has quite a large historic area just made for wandering around. The area around the basilica is very touristy. As you go uphill it becomes a lot more pleasant, and you may walk blocks without seeing another tourist. I hope you'll have time to do that. There are some large escalators that take visitors uphill from the large parking lots. I don't know whether there's anything to help a pedestrian starting from the train station other than the bus. Time permitting, I'd see the basilica and then get either a bus or a taxi up the hill and walk down.

You can take a look at the train schedule here: Deutsche Bahn website. To see fares you'll need to look at Trenitalia.com or one of the third-party sellers like loco2.com. The latter will probably tack on a service charge.

Trains to Assisi are regionals. The fare will be the same no matter when you buy the ticket. You can make a travel-day decision if you want to, but allow plenty of time to buy the ticket. Normally the vending machines (English option) are the way to go. In my experience the lines at the staffed counters are not usually long, but they can move surprisingly slowly.

There are no reserved seats on regional trains, so the trains to and from Assisi cannot sell out.

Thank you! Your information is very helpful. We know it will be a full day, but we're so looking forward to it!

If you take that early train, I suggest that you trot right out of the train station - don't dawdle - to catch one of the two (or so) taxis that tend to meet the trains. Since your time is so short, take the taxi (don't fiddle with bus; taxi will cost approx. 15 euros) to the "top" of town, Piazza Matteotti, which is just inside the walls. From there, you will literally walk DOWN through the town. Many of the recommended restaurants (on this forum and in Rick's Italy book) are right about midway through town. The wonderful free "Assisi Town Walk" in Rick's "Audio Europe" app actually starts up at Piazza Matteotti, then guides you down through town, ending at the Basilica. From the Basilica, you can walk further down (about 5 minutes) to the large open Piazza Unita d'Italia, where the buses and taxis congregate. If you START at the Basilica, your walk will be all day uphill. You can do this all in a leisurely 8 hours - or less. I do suggest that you purchase your return ticket in Rome, as it will be good for a 4-hour period, and you won't risk having to deal with one or two sometimes cranky ticket kiosks in Assisi.

Thanks for the great tips!

One thing that surprised me when we took the direct train from Rome to Assisi is that the platform in Termini is waaaaaaaay past the main platforms, to your left as you're facing all the main platforms after entering Termini. "Waaaay past" as in 1 kilometer away (more than half a mile). And no bathrooms down there, so be advised.

Definitely do the taxi on arrival in Assisi. Do not tip the driver, even if he heavily hints that you should. 15 euros is already an outrageous price for such a short drive, but they have a monopoly on it.

Good to know! Thank you!!