Florence to Ravenna

A day trip is doable for both from Florence. No rental car advised.
Siena is closer (75 min by bus or 90 min by train). The bus is preferable since it drops you in the historical center on top of the hill. Two bus companies to choose from:
https://orariautobus.org/extraurbano/autobus-da-firenze-a-siena/#gli_orari_autobus_da_firenze_a_siena
If you choose the above Busitalia/Tiemme bus the station is on Via Piazza Caterina da Siena, across from the train station, Bar Deanna side.
https://www.flixbus.com/
If you choose Flixbus, choose the bus stop at Piazzale Montelungo, near the station next to the Fortezza da Basso (accessible from the train station, via a long walk on platform 16).

Ravenna can be visited by train (one train change in Bologna), although the trip is longer (2 to 3 hours depending on which trains you select), so you will have only limited time for Ravenna, but still doable if all you need to see is the mosaics and the city center. train schedules below:
www.trenitalia.com

Would like to visit Ravenna to see the mosaics. Does a day trip from
Florence to Siena make sense or is that too short a time for Ravenna?

I'm confused
Are you asking about a day trip to Siena OR a day trip to Ravenna? 2 diff locations, not anywhere near each other.
Day trip from Florence to Ravenna is about 2 hours+ by train each way- change in Bologna- that would be too far as a day trip for me.
Day trip from Florence to Siena is easy by bus (or train).

Will be staying in the countryside south of Florence the last 2 weeks
of June.

How will you get around in "the countryside"?

After Christine's reply I realized that I may have misread your post. If you are staying in the countryside south of Florence, I assume you are not in Florence proper, and will probably have a rental car already. In that case, a day trip to Siena is best done by the rental car you will have, since you are likely on the way to Siena anyhow (south of Florence).
Ravenna by car from south of Florence is not a feasible day trip. You are looking at a tiring 3+ hours drive+parking each way. Driving to Florence + taking the train would mean still 3 and 1/2 hours trip at least. I wouldn't attempt Ravenna, unless you were staying downtown Florence, therefore within a short walk to the train station.