Tower wouldn't be open since the fed gov is closed, but the building - yes. Go sit in the hotel lobby and see the vast space. https://www.nps.gov/nama/learn/historyculture/old-post-office-tower.htm offers information, https://www.gsa.gov/historic-buildings/old-post-office-washington-dc too.
Actually the entire building is open. It's a hotel now. The Tower belong to the hotel. So your free to stop there and have a drink.
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