While someone else may have a better/different suggestion, here is what I would do in your situation.
First, acquire a secondary wireless router.
Have this secondary router plugged into your normal router so that it has internet connection, and give you a completely separate 'wireless' network that visitors can see/connect to that's aside from yours.
Then just lock down the router so that anyone connected to it can only
access, for example, port 80 and 443 for internet access. This would keep your network/devices safe from snooping, etc.
While most wireless routers may not have the option/ability to lock down ports and only leave certain ones open (again some do, but then some don't), you can always look into installing different firmware onto the router that would give you such ability. Check out Tomato Firmware
that these firmwares have listings for what routers they have specifically been tested to work with. You don't want to just go ahead and install such firmware on a router without making sure it's compatible first...otherwise you may have a dead/non-functional router.
Also, an example of using DD-WRT to accomplish what you are looking for can be found in this guide
. (Look for the section labeled "Block all traffic except HTTP HTTPS and FTP
Hope this works/helps with what you were asking for!
Again there may be better suggestions from someone else, but just wanted to give you my input...