I can't recommend anything at the moment in the way of plays as the season hasn't started.
You get the best view of the action standing, but you will have to stand for the whole 2-3 hours (as if they let you sit on the floor fire regulations would cut the capacity to an unsustainable level) and you will get wet if it rains. If you stand make sure that you have eaten enough during the day - there are regular cases of people fainting and having to be carried out which is embarrassing for the person and disruptive to those around them.
Seating is more expensive and doesn't give as good a view except from the very best seats, but you get to sit down and don't get wet. Be warned that the stairs to the Middle and Upper Galleries are long, although not steep or scary, and there is very limited elevator provision reserved for the seriously mobility-impaired. Seats in the front rows of the Upper Gallery are not recommended for those with serious fear of heights.
If you just want a tour I would suggest going in the morning - during the afternoon and evening the theatre itself is strictly reserved for people with performance tickets.