Playing guitar is definitely the best way. Here's what I do:
Create a Sim with the charismatic trait so you can get the Super Popular wish. Then have the Sim play guitar in front of the theater; this is where there will be the most Sims passing I think. All the Sims that stop to listen will become acquaintances. Do this for a few days in a row until the Sim has 25 acquaintances (the charisma Celebrity challenge; you may need 1 charisma skill point to get this) which will give a boost to relationships. Also pay attention to wishes so you can collect some happiness points and get the Attractive reward; another boost to relationships.
With both the Celebrity challenge and the Attractive reward everyone your Sim meets is an instant friend. Go back to the theater and play guitar some more and your Sim will be making loads of friends without ever talking to anyone.
I can complete the Super Popular lifetime wish in about a Sim week this way.