Lighthearted, obvs.
We ran out of bubble bath, and my 2yo dd thinks baths without bubbles are akin to waterboarding. In desperation, I squirted our mild family-friendly hand soap into the bath. It smells of fruit salad sweets! It froths up pretty well and I've been using it ever since.... Probably won't get around to buying bubble bath again.
An acquaintance grew up using fairy liquid for bubble bath, which would probably get a lot bubblier, but is perhaps not as kind to skin.