Around an hour's drive northeast of Wiesbaden is Saalburg (near Bad Homburg). It's an old Roman fort which is worth checking out.
It is fairly near the Hessen Park (maybe ten minutes away). My general advice....if you are going up for one.....go early enough to check out both the Roman fort (2.5 hours required) and the Hessen Park (four hours required). I did the Hessen Park six months ago, so this was a separate trip for me.
Saalburg was designed as a Roman fort around 81 AD. The Romans used it primarily to protect the local trade route that went through the area. The historians say somewhere around 160 to 500 Roman troops were there at various times.....with a maximum of 1,500 civilians as part of the population as well.
Saalburg was abandoned around 260 AD when the Romans packed up and left. The fort fell into disrepair and eventually was rediscovered around the 1850s by a Nassau (what came before Hessen) group who had interest in Roman history.
The entry fee is five Euro per adult. Kids are around half that. There's a decent coffee shop/restaurant there.
There's a fair amount of walking and I'd skip it in the winter months. Overall, it's worth a trip if in the Wiesbaden/Mainz area.