Last night I visited the paid sites (Prague 1&6) as well as free sites in the center, using one-day ticket. I have not yet visited the free sites in Prague 6. Given that rain is in forecast for Saturday night, tonight is likely to be a very busy night.
Some FYIs for family visiting with kids & one-day ticket holders:
1) Visually interesting sites for kids in the center -- videomapping (old town square, free), Mercedes-Benz sponsor site (Clam-Gallas Palace, free), 4 paid sites (Kostel sv. Šimona a Judy, Právnická fakulta, Mirror Chapel of Klementinum, Divadlo X10)
2) Crazy long line -- I intentionally waited until after 21.00 on Thursday night to visit the paid sites in the center to avoid lines. People who had visited Jízdárna Savarin (paid site) earlier in the evening had warned me about the long line at this site, so I decided to go late. I was stuck in line for 50 min (22.50 - 23.40). The non-VIP line did not look long, but only small number of people is allowed in every 10 min, most of them are VIP tix holders through VIP line, so the non-VIP line barely moved. Many people in non-VIP line dropped out. Before allowing into the site, staff briefed my cohort (about 10 people) to inform us that it is pitch black inside and we were asked to turn off the phone and put it in a black bag they provided. No kidding, it was indeed completely dark with light flashed for 1 second every 20 second or so, my cohort could not see our way to move beyond the entry point, so we were just clustering at entry point, most of us decided to get out after a minute.
3) The paid-sites worth standing in line for -- the only paid site I did not visit last night is the inconveniently-located Old Wastewater Treatment Plant. This year I find Divadlo X10 the most interesting, be aware: 1) need to walk 2 flights of stairs to basement, there is no elevator, 2) the very interesting video presentation is longer than 10 minute and people are spending rather long time at this small site. I stayed 20 min. On a busy night, the time in line could be long. Kostel sv. Šimona a Judy, Právnická fakulta, Mirror Chapel of Klementinum are all moderately interesting.