One of the most well known solutions is a variation of your solution number 3 that uses a pseudo element instead of a non-semantic html element.
It goes something like this...
.cf:after {
content: " ";
display: block;
visibility: hidden;
height: 0;
clear: both;
}
You place that in your stylesheet, and all you need is to add the class 'cf' to the element containing the floats.
What I use is another variation which comes from Nicolas Gallagher.
It does the same thing, but it's shorter, looks neater, and maybe used to accomplish another thing that's pretty useful - preventing the child elements' margins from collapsing with it's parents' (but for that you do need something else - read more about it here http://nicolasgallagher.com/micro-clearfix-hack/ ).
.cf:after {
content: " ";
display: table;
clear: float;
}