I should preface this first by saying that I am extremely unstylish and a bit butch, my standard summer clothes would be cargo shorts and a T-shirt. I also have extremely ugly and problem feet, so from experience the more stylish sandals look pretty awful and out of place on my feet. So in short, I am looking for the type on sandals your dad might wear.
In summer I usually wear flip flips (but get terrible blisters from the toe post, and there's not enough foot support) or trainers (but my feet get sweaty).
Given a choice between ugly walking sandals eg www.sportsdirect.com/karrimor-amazon-ladies-sandals-188030?colcode=18803003
or birkenstocks (not the cool ones....the geography teacher ones...)
www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjIge6f643MAhXKfhoKHVdcDewQjRwIBw&url=https%3A%2F%2Fwww.pinterest.com%2Fexplore%2Fbirkenstock-sandals%2F&psig=AFQjCNFGvSC9nz6cDtoufSS9i2Urib3mCA&ust=1460713262985801
Which do you think are least awful? 
I already have the birkenstock (but can return them), I think they look ok with trousers but questionable with shorts. Walking sandals are usually even uglier, but perhaps more practical.
Maybe I should just give up and sweat in my converse for another year 