When people try to think of picnic places, they usually think of UP Diliman, and while there are a lot of good places to choose from, the area near Quezon Hall would be a good pick.
Most people like to picnic around the sunken garden, but the ground there is on an incline so it's not so fun to have bags, food, and your friends rolling down into the depths below. Plus, everyone passes by there and it's where a lot of the jeeps stop. Sure, you may be closer to the tindahans and public transport, but is that what a picnic is really about?
The fields behind the Quezon Hall (that's the building straight ahead if you're on University Ave., where the Oblation is) facing the College of Music are level and shaded with trees. If you move farther away from the street, it's a perfect place to lay down a mat and enjoy the environment's peace. A lot of prenups are done in the area and with good reason--even though you're still right smack in the center of urbanity, you still get to enjoy the picturesqueness of nature around you. You can even play small sport activities or have canapes and a bit of bubbly and just have a quiet afternoon there.
The stepped UP Amphitheater just behind Quezon Hall could be fun to traipse through, as I wouldn't know what else you would do there. Take whimsical pictures I guess? And you could also just enjoy Nakpil's architecture in Quezon Hall itself. If you could get the chance to go up to the bridgeway of the building (as I'm not entirely sure they'd let people in there), it has a nice view of the academic oval and the joggers and the jeeps passing by. You could even greet Oble a good day! Just don't take a photo with him if you're still a student though. The superstition goes that you might not graduate on time if you do. ;) read more