You can do relatively simple same-day runs from DTW on Delta for around $250 on August weekends.

Atlanta and Baltimore consistently seem to be the cheapest; Atlanta generally requires a very early departure, so perhaps paying the extra 10 bucks for later departure to BWI might be easier. However, on various dates New York, Chicago and others are not all that much more expensive, and if you want to put in a few hours of sightseeing or culture maybe you can select a destination on that basis rather than merely choosing on price (as I have done in the two illustrated examples below).

If you can stand to add in a night at a hotel (maybe turn this into a bit of a mattress run too, eh?), you can bring the cost of an ATL trip (out Saturday evening, back Sunday morning) to $168.

You only need to book a Basic Economy ticket; you do not need luggage (presumably) and they're relatively short flights so no need to pick a seat. Just remember to book your ticket WITHOUT logging in to your Delta account. The whole purpose of booking this ticket is as an FB rescue mission, so you must use your Flying Blue number ONLY. Be warned that there is a danger that, if you use your Delta login/account AT ANY POINT in the research or booking of this ticket, or later on, in the management/maintenance of the booking, including checking in online or at the airport or identifying yourself to a staff member as a Skymiles status member, the whole purpose of the trip may be defeated as the miles may end up going to your Skymiles account rather than to your Flying Blue account. This also means that you MUST forego all your usual Delta status benefits; don't log in or give them your number or show your card at the desk to get put on an upgrade list, to get extra baggage, to be allowed to pick a seat (I'm not suggesting that your Skymiles status would get you any of these things on a Delta Basic Economy ticket - I have no idea what status perks you may normally expect on a Basic Economy ticket - but you MUST forego them) or to have any other Delta-specific benefits that your Delta Skymiles status may normally give you; you MUST experience this flight as a regular Flying Blue customer. And we have far more reports on these boards about the issue of people with both Skymiles and Flying Blue accounts, where they (sometimes inadvertently) "pollute" their booking with the other account details, and the miles end up going to the wrong account. So if you decide to book a trip solely to attempt to rescue your Flying Blue miles, then for the purposes of that trip, you would be advised to completely forget that you have a Skymiles account, and under no circumstances should you seek to obtain Delta Skymiles status benefits when travelling on this ticket.

It may be safer to book this ticket from a third party agent (and price comparison websites may even give you a discount on what Delta themselves are charging); that way there is no risk of you "accidentally" getting logged in using your Delta/Skymiles account. And please resist the temptation to look up your reservation on the Delta website, again in case you "accidentally" get logged in there and they associate the booking with your Skymiles number without your realisation or actual consent.

I would also strongly caution against doing something like substituting in your Skymiles number after flying the outbound, in case you were thinking that just posting that first outbound flight to your FB account is enough, and now you'd really prefer to post the return leg to your Skymiles account. There is a very real risk that, given that the miles have not yet posted to Flying Blue, the change of number to Skymiles mid-journey will cause ALL miles to post to your Skymiles account, and therefore your Flying Blue account will not get any miles and the journey would have been completely pointless.