Lucky Karmz and I strolled the grounds since it is one of the free things to do in Pittsburgh. For me, the best part was the Cafe. The Car & Carriage museum is neat if you like to see old carriages and old school cars - that kept my interest for about a minute before I was ready to move on. There was no admission fee so I really had no excuse.
There are beautiful flowers and trees everywhere - perfect place for an event/wedding. The centerpiece is Clayton, the home of the Fricks. We admired the exterior as we didn't have time for the $10 interior tour. The Greenhouse looks cool from the outside, I didn't go in. The Art museum suggests a $5 donation but I wasn't particularly interested in Frick's personal collection from what I've seen online.
You can cover the grounds in about 20 minutes without going in any of the places. Another 15-20 minutes for the car and carriage museum if you plan on reading up about them. Factor in another 1-1.15 hours for afternoon tea or lunch/dinner at the Cafe. You could spend a good half day here if you did the Clayton house tour.