the number e is by definition the number e such that the integral of 1/x from 1 to e equals 1. thus any method of approximating integrals allows you to approximate e. i.e. if yuo show that the integral of 1/x from 1 to 2.7 is less than 1 you have shown that e is greater than 2.7
another nice way is to use the taylor series for e^x, and plug in x =1, which gives e = the limit of the series 1 + 1/2 + 1/6 + 1/24 + 1/120 + ...+ 1/n!+...
My freshman calc prof gave as an exercise to prove this way that e starts out as 2.718281828... but i never completed it. i could not believe in those days that math took that much work!