Frankfurt plans- should I alter?

You should clarify - am or pm for times, and which way is this, on your outbound or your return flights?

If, as I think you're saying, you arrive in FRA on a Friday and you have to overnight there before continuing on the next day...either 8:35 pm or (even worse) 8:35 am, that's a connection I would not have booked. You are basically using up a day of your trip for this. I don't know about you, but I think for most people, the commodity that is most scarce is your time. Most Americans' trips to Europe are too short. Burning a day (or a day and a half) with an unwanted stopover is not doing yourself any favors. I would work hard (and even pay a bit more) to avoid that.

That said, nothing wrong with Frankfurt (and no offense to those who like going there). I do often plan a stopover either inbound to Europe or outbound on the way home (for other reasons). If you really want to spend a day or two in/around Frankfurt, fine, but if your trip is really to someplace else and you don't have plenty of time there...consider fixing your routing to better maximize your time where you really want to go.

Just my 2 cents.

I don't have any great suggestions but I did want to suggest to David that he reread your post, as you obviously mean 8:35 as Friday morning (otherwise there would be no time to allot to your Day 1 activities) and your departure flight is stated as Sunday, giving you all of Saturday to plan Day 2 activities.

If nothing else you can always spend Saturday enjoying some great German beer!

I vote for Marburg if you want to see the half timbered houses, just a very nice town, architecturally, historically, and also a famous university town.

Have you been to Europe before? You are unlikely to get "into" a hotel room at 10AM. You can leave your bags with the bellman until you come back from your sightseeing, of course. August is vacation time in Europe, and tourist sites can be very crowded.

Frankfurt downtown has its own small Fachwerke section. If you don't want to go to downtown Frankfurt and want to see a more real village, the S-Bahn will take you to Frankfurt-Höchst, and you can check the walking distance to the old town using Google Maps. That could be an arrival day activity, leaving you dinner in Frankfurt (or Hochst.) But you will be drooping by dinnertime. Have you read up on Jet Lag? Because your profile doesn't have your home state and country, I can't be sure you're coming from the US.

In Germany and Austria I was allowed to go to my hotel after check-in provided the room was not used the night before. If you arrive at 10 AM and if your room is available since it was unoccupied, you can go to the room after check-in.

Since you arrive at 8:35 AM, you conceivably be in Marburg by 13:00. on day Day 1. I could. Keep in mind that not every one gets jet lag upon arrival.

Yes. We arrive at 8:35 am Friday and leave 9:15 am Sunday. We have been to Europe a number of times. Sometimes we have jet lag sometimes not.
Last year we flew from Houston to Istanbul to Maldives. Oddly very little jet lag. (Husband works shift work I think that helps)
We have been able to check in before, we also have checked our luggage with the bellhop or the like.
Frankfurt is our short stop prior to going to Russia.
Marburg does look beautiful.

The old part of Frankfurt is absolutely the most interesting to me, thank you.

Thank you for your advice.

The Frankfurt on Foot tour you mention runs every day at 10:30 am. You’d have to hustle to make that one, after putting your luggage in a locker at the train station. They will do private tours at other times if that’s an option for you. Otherwise, the city tourist office runs English tours at 2:30 every afternoon. See Frankfurt-tourismus.de

Marburg is an hour on the train from the main station, plus your time to get there from the airport and deal with your luggage (and another hour back). I think that’s a little far for the time you have. I’d go with a walking tour and then a 10-minute local transit ride to Frankfurt-Höchst, a part of the city but full of original half-timbered houses and a castle. Have dinnner there, wander around. Close the evening with a walk along the river?

Edit: you could actually stay in Höchst too. It’s an easy trip back to the airport.