Commuter rail from Auburndale station. There’s a lot there, it runs on schedule. Is faster and more comfortable than the T, and you can walk from Woodland or Riverside if you miss the train back.
On a Wednesday the lot will be full unless you’re getting there fairly early.
I live in Lower Falls, about a half mile from Riverside. I normally take the D line in and walk from the Fenway stop. But I don't mind being on the green line for 50+ minutes. If a long time on the T is daunting, you could park at Riverside and walk up to the Auburndale commuter station. It’s a fairly straight shot and there is a train at 11:42 that’ll get you to the Yawkee stop (directly across the street from Fenway) at 11:56! 12 minutes vs about 50 sitting on the green line might be worth the half mile or so walk.
If you do pick the Riverside park/Auburndale train option, ignore the directions Google will give you to get to the station. It sends you to the far entrance instead of the closer one. Walking out of Riverside, take a left on Grove St and take Hancock St. When you get to Woodland Road, take a left and go a block until you get to the bridge over the pike. There are stairs down to the platform and you’re there. It’s just over a half a mile.
For landmarks, there’s a place called the Walker Center at Hancock and Grove (big red sign) and there’s a church at the corner of Hancock and Woodland. It’s a bit of an uphill walk until you get about halfway up Hancock from Riverside but even with the walk you’ll be at Fenway faster than the green line.
And on the way back you’ll have options if you miss the return train. Just make sure when you're looking at the outbound schedule that the train actually stops at Auburndale or your return trip might be a wee bit complicated...
There is nothing like a "what's the best way" question to get a bunch of New Englanders going.
Posted after I started my proof of same...
Edit: blarg - I way overstated the travel time via green line. 50 minutes is my door to door, which includes the 1/2 mile walk to Riverside and the walk from the Fenway T stop to Fenway itself.