Going to Germany in May. Can we wing it when we get there? Book hotels or airbnb's as we go?

If you are two people who aren’t picky and don’t have a limited budget, you have much more flexibility with lodging. In May, you will probably have plenty of options available.

Hotel alternatives would be apartments or a room in someone’s home. You can book these through Airbnb, VRBO, TripAdvisor or booking.com. Beware of fees (cleaning, admin, etc). They can add a lot to the cost, especially if you’re only staying a night or two. We are a family of 4 and we stay at least 3 nights at each place, so we like apartments.

Hi Andrew,
How did your last trip work out for you?
If you were happy with winging it and the resulting holiday that you had, then
being spontaneous on your German trip
likely will work out for you. You can save a lot of money on train tickets and hotels by booking them in advance. Trivago.com is a good resource for this. VRBO is great for apartments as is Booking.com
When all is said and done— winging it is usually going to cost you more in the long run as the better-value accommodations and discounted train tickets like the”Sparpreis and “SuperSparpreis” tix ( see Deutsche Bahn’s website “Bahn.com”) get snapped up by those who do plan ahead. If that’s not a concern for you, though, then being spontaneous may be your travel style.

Sparpreis and Supersparpreis tickets will sell out, but if you use a rail pass there is no need to worry as very few trains in Germany require a seat reservation and with a pass you can basically just board any train you feel like at the station.

Nothing wrong with winging it! You are going to Germany for experiences, so go where your heart wants to take you.

There are always good hotel and rooms options available. Bring a good guidebook like Rick's or Michelin's so you have a ready list of vetted options to go through when you decide on your next town at the last minute. You can also visit the town's TI when you arrive and they will help you find a place.

We would just as soon rent bikes and ride through the countryside or
walk around towns and cities window shopping. going to Farmers markets
etc.

You're right, no lines for that sort of thing. But even now there are "virtual" lines forming for accommodations in the more popular tourist destinations. I'm pretty sure I voted for "hub and spoking it" on your previous post, and I will do that again here, even if you end up in places no tourist has ever ventured. This tactic gives you a lot of flexibility with minimal constraints. You don't have to spend any of your "biking - walking - window shopping" time hunting down accommodations - or waiting in line at the TI to help you find accommdations - that might end up being poorly located or unsuitable for other reasons. And every single morning, you can wake up, assess your personal mood and interests over breakfast, and then head out to "wing it."

If this method suits you, don't give it up just because a handful of people don't see it your way. It's a great way to experience Germany, one that many travelers never try out because they're spending 2 weeks trying to hit all the long lines in all the places that Rick Steves and other tour guides / tour book authors recommend.

Also, is airbnb the best way to book hotel alternatives?

Airbnb-like rentals to tourists are outlawed in Munich and in other cities where housing shortages exist. Because of your interests, I suggest you pick maybe 2 "hubs" and book a "Ferienwohnung" (vacation apartment/rental) for a week or so in each hub - that will normally get you a reduced price. Airbnb handles only a fraction of the rental options in Germany. There are similar organizations there but the most productive source is the local or regional tourist authority. The Black Forest might be your kind of place. The Black Forest tourist authority lists 3,600+ rentals:

https://www.schwarzwald-tourismus.info/stg/ukv/result?tt=be92299d2f80c5e5a89750067bd761bb

BTW - many Black Forest towns offer free regional train travel to guests via the KONUS card - so travel on your "spokes" would not need to be pre-booked, or even paid for. The same is true in a few other places in Germany, like Boppard am Rhein:

https://www.boppard-tourismus.de/en/accommodation/

Your hubs should be located such that you have several feasible day trip options at hand. Because of your outdoor interests, I think at least one of your hubs should be a small town, where you wouldn't be obliged to make a day trip somewhere else just to take a bike ride or whatever. If you pick places with good railway connections, you can take spontaneous day trips at very good prices using day passes. At the rail station, buy a state-wide day pass, like the Rheinland-Pfalz ticket (€31/ 2 adults,) which gets you anywhere you like between Bonn in the north and Karlsruhe in the south. It's a huge area to explore.

https://www.vrminfo.de/fileadmin/data/pdf/2017/RLP-Ticket_streckennetz.pdf

Bavaria has even more territory - too much to explore in just a day, actually.

https://bahnland-bayern.de/files/media/bahnland-bayern/service/streckennetz/beg_streckennetz.pdf

Most local train systems have provisions for taking a bicycle on board.

So you might have a little research to do to find the right hub town and to collect a few options for outings. But aside from your flight and maybe your transfer between hubs, you will not have to plan a single day's activities or keep any particular schedule. And there are people here that are willing to advise you on the hubs and the spokes.