One of the most useful pieces of equipment for a field naturalist is a hand lens. These come in a range of magnifications and prices. I would recommend starting with an inexpensive 10x lens, such as this 10 x 21mm Hand Lens, as reviewed by BBC Wildlife magazine.
You can also buy lenses with more than one magnification. I use this double lens which has both 8x and 15x magnification.
Probably worth bearing in mind that multiple lenses such as this tend to be heavier and more expensive.
Other useful field equipment
It's often useful to have something to take notes on, and something to write with (a pencil will be better if it's likely to rain). A dedicated field notebook is useful, but I quite often just use scrap paper. One possibility if you want to be prepared for wet weather is a waterproof notebook.