Ullevålseter cafe is located at the half-way point on the classic Frogneseter - Ullevållseter - Sognsvann hike. They serve up delicious rolls, waffles, and cinnamon rolls, so it's a perfect place for a lunch spot when out hiking. It's also a connecting point between a few different ski tracks in the winter, and a nice place to drop by to warm up when out cross-country skiing.
To hike Frogneseteren-Sognsvann, take the metro to Frogneseteren (end of line), and follow signs to Ullevållseter. Stop for lunch, then keep going to Sognsvann. Take the metro back to the city. The hike is 8k, and would take about 4h.