Editing session values from blocks

My site www.dance-ottawa.com is getting more traffic, as well as postings and users from outside Ottawa.

Its occurred to me that it would be handy to have a block on the site that lets me set session variables for user location and proximity range for the current visitors. I could then use these values in location proximity searches to help filter down the content.

I need session variables, because I don't want people to have to log in in order to select their home location or range of interest. I don't have a 'current user' in that case (its user 0 for all anon visitors), so I think this proximity filter data will have to be stored in session data.

If somone could show me how to make a block that displays a single default session variable, and lets someone edit it and save it back in session, then that would get me rolling i think. I think from there I coujld probably figure out how to connect it to the location module.

Thanks