Travis Kelce 's traded the garden where he got engaged for the gridiron ... 'cause he was seen out just days after he and Taylor Swift made their announcement.

The NFLer hung out with his big bro, Jason Kelce , chatting it up on the sidelines of the University of Cincinnati and the University of Nebraska.

Check out the pic ... Travis and Jason are both repping UoC -- they both went to Cincinnati before beginning the pro careers that will land them both in the hall of fame one day.

Travis hasn't been seen out much recently ... and, this is the first glimpse we're catching of TK since we found out he got down on one knee and asked T-Swift to marry him. Gallery Launch Gallery

As you know ... Travis popped the question garden in Lee's Summit -- surprising Swift on their

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