If you only have a few hours to spend in Aberdeenshire, there are a few must-see attractions that can easily be done in that timeframe. One option would be to visit the famous Dunnottar Castle, which is about a 30-minute drive from Aberdeen. Depending on how long you want to spend exploring the castle ruins and the surrounding coastline, this could take between 1-2 hours.
If castles aren't your thing, you could head to the Aberdeen Maritime Museum, which is located in the city centre. This museum tells the story of Aberdeen's historic relationship with the sea, and features exhibits on everything from shipbuilding to fishing to oil exploration. You could easily spend an hour or two exploring the various displays and learning about Aberdeen's maritime history.
Another option would be to walk around the Old Aberdeen area, which is located just north of the city centre. This area is home to the University of Aberdeen and features a number of historic buildings, including St. Machar's Cathedral and King's College Chapel. You could spend an hour or so wandering around the cobbled streets and taking in the sights.
Finally, if you're looking for something a bit more active, you could head to the Hazlehead Park, which is located on the outskirts of Aberdeen. This park features a number of walking trails, as well as a golf course and a children's play area. Depending on how much time you have and how much you want to explore, you could easily spend an hour or two in Hazlehead Park.