I would greatly welcome any and all information regarding Thessaloniki. We are headed there for 5 days next month. Not a lot of information easily available. Thank you in advance for your help.

Do you have a good guidebook? IME, the Rough Guide provided quite a lot of information about Thessaloniki.

You might also find some useful information in my trip report. The section on Thessaloniki begins with post # 174. (Also see the end of post # 168 re: Vergina.)
With Gratitude for a Glorious Solo Month in Greece

I assume you've already got ideas of what you want to do when you visit (or you wouldn't be going) so guessing you've already got the basics such as these sites provide from a simple internet search:Are you looking for specific things of interest?

fwiw, we spent 6 nights there last year. My first impression (and having come from a very rural area) is that it was busy and a bit gritty, but after a very short time really settled in and enjoyed our stay there and certainly didn't run out of interesting places to visit. A friend highly recommended a visit to Vergina but unfortunately we didn't make it. The same friend had her passport stolen on the bus from the airport so watch your valuables (as goes for any city) - she freely admits she wasn't as careful as she could have been.

What are your plans:
city with white tower, Atatürk museum, archeological museum, etc?
day trip to one of the stunning Chalkidiki beaches?
Vergina and Verria?
Pella and Naussa?
archeological site of Philippi and may be waterfalls of Kavala?
Lake Kerkini National Park?
archeological site of Herakleia Lyncestis (North Macedonia)?

Thank you for the report, links and excellent suggestions.

I ordered. Northern Greece: including Thessaloniki, Macedonia, Pelion, Mount Olympus, Chalkidiki, Meteora and the Sporades, 9/2020 by Dana Facaros. It won't arrive until next week. We are leaving on 9/6 for Copenhagen and then heading to Thessaloniki.

We have not planned anything yet! This is a very last minute trip.

We are an older couple interested in cultural and historical sites, Jewish history, food, hanging out in cafes, lovely beaches, maybe small tour groups, markets of all kinds, museums, strolling through interesting and beautiful neighborhoods, We are foodies interested in great restaurants.

Moreover we are looking for some hotel recommendation in the city center.

Thanks again for the warm responses.

You might take a look at the Caravan B&B:
https://thecaravan.gr/

For restaurants, you might consider Sempriko -- a casual place with very good food.

If of interest there is a multi site tourist card which you can purchase. There is also a public bus which takes you past/near many of the central tourist sights. It's worth getting a little familiar with local buses and their tickets. Even though we didn't use them a great deal, they did come in handy sometimes, especially as we stayed up in Ano Poli.