I like this little chart. It's got some good basic tips to go by as well as some good bases to cover. Hanging out and observing can be excellent socialization. The pup doesn't always have to be directly involved in something to work on socialization. I played sounds for my pup when he was younger because i wanted him to get used to lots of things. I had some recordings (or found them online) of crazy party sounds, carnivals, thunderstorms, etc. It wasn't my main form of desensitizing to sound, but it was a good stand by if I couldn't get out very much.
I'm a believer in safely exposing the puppy to as much as possible. My pup went with me to every public place possible. We went to the dog park and sat a distance away, outside the fence, on concrete to get used to other dog sounds and seeing dogs running around. But other places like pet stores, patios, friends houses, outdoor malls, even drive through's are great places to take them. I even took mine to sit outside fire stations, police stations, and hospitals to see people in uniform, radio crackle, sirens, people in scrubs, walkers, wheelchairs, and with canes.
I just go until the puppy gets tired. If I see him not showing as much interest, or showing any type of discomfort (ears back, tail tucked, excessive licking or yawning) then I take him back home, make note of what I was doing and how long we had been out, then tried again another day. I also usually pick two days a week to have "off" days. We'll go on walks and play like normal, but won't do the heavy socializing at super stimulating places. It gives the puppy a chance to recuperate a bit.