I've been to the Los Altos Fall Festival many times and it's fun both as a participant and as a visitor. There is a lot to do and you could easily spend 3-4 hours at the Festival if you really do all the stuff they offer. It's a bit like a smaller version of the Art & Wine Festival but without the crushing crowds and frustrating lack of parking.
The Kids Alley is a lot of fun, especially for younger kids. They have a small train that kids and adults can ride. The games are a good challenge for the 8 and younger crowd while the crafts keep the older kids happy. You could a lot of time and very little money with the kids here. As with most of the downtown events, this is a great place to meet local parents while the kids have a good time. If you're new to Los Altos, stop by and make a few friends.
The wine tent is a chance to try wine from the local area at a reasonable price. There's usually a nice spread of cheese and crackers (top quality) available as part of the admission price. Winery owners and staff will happily answer questions about their products so you get the fun of going on a wine tasting tour without having to drive.
For anyone who likes cars, you'll get a nice mix at the car show put on during the Fall Festival. While you have the usual cars from the 50s and 60s, you'll also see older cars and a few newer "classics". I've never really counted how many cars they have but I'd guess it's at least 25 or so. The owners are usually on hand to answer any questions and you can get "up close and personal" with almost all of them.
There are a lot of booths but not too many and the mix is a little more interesting than the usual art and wine vendors. Many products offered are unique to the time of year which is nice when shopping for decorations and gifts. The food is fairly typical but you also get restaurants from downtown Los Altos participating in the mix.
As with most festivals, there is fairly continuous entertainment from bands that put on a good show. I really like the bands they have because they tend to play songs people know, play them well, and not so loud you can't hear yourself think. There's good much for adults and kids alike.
10AM - 5PM