Hello, lpage53, and welcome to the forum,
It seems like you have a few things going on here - you want ride local trains and then hop off in a small town and explore it but you're not even going to have a full day to explore the town you choose. In general the small towns off the main train lines don't go onto to serve the major lines so heading to a small town means backtracking back to main line and train changes to get back to travelling. This is a wonderfully romantic idea but I would generally recommend knowing your destination when you buy tickets and having a room secured - even in October - in most places before you land there. Even if rooms are available you'll end up wasting an hour or more finding one versus spending your brief time in town exploring. Most towns look alike from the train station so the view from the station is no way to judge a town.
Lucca is an obvious solution that provides much of what you're looking for so it's a pretty good choice. Choosing a random weekday in October you can reach Lucca using in 4:19 using local and Intercity trains from Venice @9:10 for ~23e a person booking early and second class on the intercity or a max price of ~48e for 1st class booked day of. Or in 4:49 @9:40 fully regionale for ~27e a person.
You can move onto La Spezia @8:30 the next day in 1:38 for ~14e on a regionale and Intercity. Or if you want to explore Lucca some more you can go @12:42 in 1:38 on two regionale trains for ~10e. So Lucca is more like 2/3s of the way there versus halfway.
On paper places like Mantua/Mantova or Cremona seem like they sort of fit the bill but getting back to a train line will add hours to your onward journey. Modena could work depending on what you are looking for. Modena is a larger city now but was traditionally a small agricultural town so the centro still feels smaller and has plenty to explore for two part days but I don't think bike riding would necessarily be one of those things.
Modena is ~3:00 from Venice for 17e to 24e a person and then about the same onward to La Spezia changing in Parma for 17e so it is pretty much halfway in terms of time and cost.
Fast trains have assigned, comfortable seats and things like WiFi and power along with being faster so there is a reason people pay more for these thing as well as the additional time savings. Booked ahead that can purchased much cheaper than the "base" fare if you are willing to trade flexibility for price. Intercity are direct trains that have classes and assigned seats that were the business class of their day but are now greatly overshadowed by the fast trains. There are modest cost savings to buying earlier and choosing second class but they are not significantly more than local regionale trains.
My $.02, have a great trip,
=Tod