And a few follow-up comments on what Penny said.
E-ticketing makes it more difficult to "play it by ear", as doing a shorter turn than ticketed for could result in you being a "no show" if you haven't been scanned at your ticketed point of origin on the second train.
Rail and station construction in most parts of the country "ramps up" in the warmer months; that is, relatively less work is done in the winter when the ground may be frozen. That said, this is Florida, where construction season should be year round. So whatever delays may be caused now by SunRail construction should not get much better or worse in July.
I presented Winter Haven as an option, but being the shortest of the connections, it is probably least recommended. The plus, if you do make it, is that you get the Tampa dogleg mileage on 92. Winter Haven is a nice station (I've used it before), IIRC a couple blocks off the main drag but not too much around. But then you wouldn't want to venture too far away anyhow given the quick turnaround.
Personally, beautiful parks and beautiful lakes don't do anything for me, though if they manage to work Wifi access into those places they will become more beautiful.
With regard to Virginia, I forgot that the Quantico station is within a restricted area, so that coupled with the shorter turnaround time makes it a poor choice.
Fredericksburg: Don't forget we will be there on Friday too, turning around from VRE to Amtrak #80. There is a station building, but I don't recall seeing it open during my past visits. I do remember some intricate ramps built to make the pedestrian underpass barrier free. So if you choose to turn there on Wednesday you will at least get to know the place well for our Friday visit. The main source of confusion there is that trains can operate on either track in either direction. Based on how many freights are in the area, and the fact that many VRE stations only have platforms on one side of the right-of-way, trains sometimes run left-handed. With a lack of station personnel to announce the proper track, one can find themselves waiting on the wrong side when the train shows up. Positive: You'll probably be standing there in Fredericksburg when the southbound Auto Train comes through. (Of course you'd also see it later on in Richmond.)
The Fredericksburg station is right downtown. So there are stores and small restaurants you could walk to. I recall also once walking to a convenience store a couple of blocks away.
I don't think Richmond will cause much suffering. There are strip centers, a large supermarket, and chain restaurants along Staples Mill Road. The longest walk is within the Amtrak property between the station and the street; once along the roadside you can walk a shorter distance to those places. A short walk north of the driveway is a McDonald's. The length of the layover is tough, but you can watch other trains come and go, and again the Auto Train probably close to 6 PM.