Tip jar

A tip jar is a spawned object that appears in The Sims games. It's commonly depicted as a spherical glass jar with a green Simoleon symbol on a piece of faded paper taped to it. When a sim engages in a certain activity, they will set this object down next to them before they start the activity. Sometimes the option to set down a jar is delineated by the words "For Tips" being a part of the command; other times using the jar is the default. Other sims passing by will come up to the performing sim, watch briefly, and then drop money in the jar. How much money was dropped can be discerned by how much money the donating sim lost, however the performing sim won't have the money added to their wallet until they pick up the jar. How much money a watching NPC will donate is usually determined by how skilled the performing sim is at whatever they're doing. When a sim finishes performing, they will go over to the jar and collect their earnings. Depending on how much money they amassed, the sim will go through either a happy, neutral, or disappointed animation, then put the jar away. Activities involving tip jars are usually indefinite and only end when the player cancels that action, or the sim becomes too unhappy to continue.

University
Three new musical instruments are added: the guitar, the bass, and the drum set. Each of these offer the options "Play" and "Play For Tips". Additionally, the expansion pack adds a "Play For Tips" option to the available interactions with the piano from the Sims 2 base game. These instruments can be played together using the "Join" interaction, but all playing sims will have their own jars and collect their tips separately.

Bon Voyage
On Twikkii Island there is the Fire Dancer NPC who will stop their performance to teach your sim fire dancing as a