Starting to put some details together. Booking a multi-city itinerary with Delta is...a challenge. Luckily, their phone customer service has been getting better and better, because these three flights, as constructed, were not bookable on their site.

May 14: MSP-DUB (66.2K miles + $55)
May 15: We need an evening flight to somewhere close to either Florence or Venice. As of right now, unless we want to try to change our stopover city, that's Verona on Aer Lingus, leaving at 15:25, so we'll need a place to hang out for about eight hours or fly one of Ryanair or Aer Lingus to Venice the following morning. Either option works, although kind of leaning towards the former, given the relative dearth of hotels (at least affordable ones) around DUB. Can book with Avios, which is probably preferable to booking with cash (6.5K Avios + $14 vs. about $150).
May 16-17: Venice, head over to Ravenna towards the evening of the 17th. No lodging booked yet but leaning towards trying to use some of our five Marriott 50K certs if possible (although, at first pass, that doesn't look super likely - the AC is the cheapest option and it's around 70K/night). Really could use some advice here, as I've never been to Venice before and my wife has never been there with a car, but we'll probably need one given the rest of our itinerary.
May 17-19: wife hangs out in Ravenna on the 18th while I go to the race. I may or may not try to stop by San Marino on the way back from the track (just to say I've been - tiny European countries fascinate me for some reason). We've booked two nights at B&B La Casa Gialla for now, for $270 total.
May 19-21: drive down to Florence, hang out there for a couple nights. Maybe add another day and catch Siena, as well, as we've now tried to go there on three separate occasions and failed on each one.
May 22: Austrian flies FLR-VIE for 6.5K United miles plus tax. There's apparently a train that takes us directly from the Vienna airport to Brno.
May 22-25: hang out in Brno with my wife's friend
May 25-28: hang out in Prague
May 28: PRG-JFK (included in the outbound price)
May 29: JFK-MSP (included in the outbound price)