This is a tough one. Valley Forge, as previously suggested, is worth more than 2 days in and of itself, and it hits your interests (except for detecting), tho outside the history, the activities are trail running, hiking, equestrian, and the like, which may take some effort to organize. It is a stunningly beautiful place, at least IMHO.
We like Phoenixville, which is an old steel town that went ghetto in the 70s-80s but since has become gentrified, and is a beautiful place to walk around, as most (but not all, unfortunately) of the historical downtown and neighborhoods have survived. Phoenixville's claim to fame is where 'The Blob" was partially filmed, Steve McQueen's first feature film, and the important landmarks remain, including the Colonial Theater, which is still operating, and usually has an offbeat film showing if you like seeing that sort of thing in an 1860s theater. Its also reported to have the most craft beer breweries per capita; whether or not that is true, who knows? It also has a very good craft distillery; if local alcohol is your thing, I can't recommend a better place. On the weekends, there is usually some sort of festival/food trucks, and the main drag shuts down to only pedestrian traffic, tho I have not checked if anything is going on this weekend. I've pulled tons of silvers out of the town, but it takes work, permission, and research.
I just like driving the backroads, and the history seems to come alive if you hit the right places. If this is your thing, just drive where the population density is low. I don't want to disparage any towns, but there are towns that don't fit the character of the county. Avoid those. St. Peters is a good town for lunch and ice cream, cool but out of the way. Nearby is Warwick Furnace, obscure and no tourist facilities, but reportedly a key ironworks for the revolutionary war. Detecting mostly by permission in this part of the county.
If you are a foodie, "The Whip Tavern", IMHO, is the best restaurant in the county. It has a Coatesville address, but it is deep in the country. Its not English-themed, its English, and it and the surrounding community are a bit unique. Detecting is old fields and old buildings, by permission only.
If you are into kayaking, Marsh Creek State Park and the Brandywine Creek are both great options. Boats and organizers for rent. March Creek detecting is by permission and state rules (may only dig with a screwdriver), and there are good possibilities there. If you have scuba gear, the lake reportedly covers and old village, which may be worth exploring. Do your research first.
As for parks, if I had to give one up, it would be Ash Park in Coatesville. This is a difficult park. Bring your A game and A machine. Very trashy and very mineralized, but the shiny is there. Its also rumored to be a high crime/drug area, but no one has ever bothered me. But, I'm simply afraid of nothing.
I could write a book on all the beauties and wonders of Chester County. We absolutely love it here. I hope this helps in some way, but it only scratches the surface, and likely misses the obvious. I don't know my plans for the weekend, but when you know yours, hit me up, and we'll see.
Good luck, and don't take a single silver out of my county