You can upgrade the hot tub with Aromatherapy. There are 4 types of Aromatherapy you can then use once upgraded.
Use Jasmine Oil - Makes sims feel refreshed, reducing the duration of Sad buffs and providing a Happy Buff.
Use Sage Oil - Makse sims feel focused, reducing the duration of Bored buffs and providing a Focused Buff.
Use Lemon Oil - Makes sims feel energized, reducing the duration of Stressed buffs and providing an Energized Buff.
Use Peppermint Oil - Makes sims feel giddy, reducing the duration of Uncomfortable buffs and providing a Playful Buff.
Update: The buffs have no duration while they are in the hot tub. But while they are in the hot tub, the buff has the blinking up arrow. I'm wondering if the longer you stay in the tub then the longer the duration you get the keep once you leave the tub. I'll see in a bit.
Second update: One of my sims isn't showing a duration while in the tub on the buff but the other three are. The initial buff appears to be 2 hours. I'll want and see if that increases over time. The buffs are also +2.
Third update: Yes, the buff duration increases over time spent in the hot tub. I'm seeing a 3 hour duration now.