As a general suggestion, I'd say pick up a book of the kind you want your document to look like and go through the points you listed and the ones I mention below.
The first thing you'll want to look at is fonts: whereas Times New Roman/Arial screams "MS Office" at you, Computer Modern seems to shout "lab manual" at your friend. In plain LaTeX you can't pull in all ttf/otf fonts on your computer directly (XeTeX, ConTeXt, LuaTeX etc. can) but as ChrisS mentioned there's a list of TeX-supported fonts and you can get your usual Times/Arial if you'd like or try something more original - it really depends on your document.
The serif/sans question can be debated to death but most books use serif so if that's what you want a document to look like I'd recommend serif.
I've been using Linux Libertine successfully recently which neither looks like Times nor Computer Modern.
The next big difference most people will note (if perhaps not consciously) between your average document and TeX' defaults is the line lengths. As Knuth and the not-so-short introduction will tell you, TeX actually follows guidelines that professional typesetters have known for ages in not making lines too long and thus the whole document easier to read.
One difficulty that you'll run into is that most non-technical books are set not just with shorter lines than "Office default" but also as a consequence printed on smaller paper sizes than letter/A4 (when did you last read a novel in A4 format?) whereas academic papers seem to come mostly with book-sized lines on document-sized paper leading to enormous amounts of white space on the pages which might be something that your friend noticed.
However, if you want to, load fullpage and chip away at the margins.
You could also consider increasing the line spacing or in TeX-speak \linespread{1.3} which will get you better readability in a long document (especially with narrow margins) and look less like your average academic paper.
fancyhdr will allow you to reformat your headers/footers. I don't have any design suggestions here except that if you or your friend find some document or book that definitely does not look like a lab manual you could try and emulate that.
Colour is another thing you won't find in most lab manuals. Bear in mind that while a document with rotated, rainbow-gradiented 3D-WordArt titles, headings in different font, size and colour to the body text and Comic Sans text definitely won't look like a lab manual, it will look amateurish and absolutely horrendous.
But a bit of subtle colour here and there for headings can't hurt - sectsty is your friend and the following pages give some examples with code:
Changing the look of section headings will go a long way to changing the look and feel of your document. Setting them in coloured boxes or with coloured rules above/below seem to be common choices.
Whitespace between and around things is as important if not more than the font/colour/size of your headings. Make sure you don't cramp things together too closely - it may not end up looking like a lab manual but it will look bad. Again, take a book you think looks good and study the way they use white space.
\usepackage{wordlike}