The Tolix chair is a great option to use both inside and out. First designed in 1934 by Xavier Pauchard with the orinigal use for hospitals, factories, offices and public parks. Today you can buy vintage Tolix (if you can find) or there are some great replica's and they are making their way as a stylish statement into a number of different spaces.
I have considered this as my extra 2 dining chairs, but I'm actually thinking more for outside for me in a bright colour. Surprisingly not yellow but in another colour for the outdoors.
images via decor8, desiretoinspire, happyhome, niceroom, onehour, teaforjoy, lonny, mattblatt