Well the best thing to do first off is to think about a shortlist of degrees you'd be interested in studying and that you will meet the criteria for entry for. Without knowing that it's difficult to advise fully. From there you can identify the areas that you'd be interested in seeing more of in terms of work experience. Without knowing that it's difficult to advise fully.
What 'looks good' on an application will not be universal. What they want to see is that you can apply yourself to a number of different tasks and adapt in an entirely new environment.
In terms of general life experience that will look good on an application and be easily achievable, look at work experience in places like Leisure Centres or Libraries - you will interact with many age groups, gain essential experience outside of an educational setting and be able to volunteer for tasks that take you outside of your normal boundaries such as reading groups, disability sessions or after school clubs.
As someone who did their work experience 14 years ago and has just been accepted into University, I found that my two weeks of work experience did very little for me in terms of setting me up for what I wanted to do. However much like yourself I wasn't set on what I wanted to do at University when arranging it, and the two placements I had gave me an excellent understanding of how to work with people that I wouldn't normally meet - i.e people who aren't your family or like your support network. I worked in two high profile, high stress placements with adults who took me on and treated me like an adult and with that trust the gave me came a hell of a lot of confidence. I didn't follow onto Uni, and instead opted for work but my WE set me up incredibly well for talking to strangers, managing tasks, sense of accomplishment and responsibility when going into the workplace which has got me to this point now.
I hope this helps.