We’ve done both, marbling with special inks, which is fun, but for a certain age group, bubble painting is considered hilarious.
Get a bowl, add a small layer of water, add some washing up liquid and some paint (need to play around with amounts to get a decent strong colour, but I’ve used both poster and watercolour paint), then take a straw and blow, then lay your paper on top of the coloured bubbles and then lift off. Your child must be able to know only to blow, don’t want an unwanted suck (yuck). The picture you get is of coloured circles overlapping, quite pretty and cheaper than marbling and with the fun of blowing bubbles 
Tanya