Jaheara,

to the Travel Forum. The Thais have cracked down somewhat on travellers casually wandering in and out of their kingdom, at least compared to how things were five years ago, and so you will need to plan your movements in advance based around the visa options available. As SteveNL says, a double entry tourist visa would give you 60 days (+30 extension) to spend in Thailand right off the bat, then you could leave after 2-3 months to travel through Laos/Cambodia etc, then re-enter Thailand for 60 days (+30 extension) to use your second "entry". To again reiterate what Steve says, if you leave Thailand then you forfeit the remaining part of that "entry" of a tourist visa - it doesn't get frozen.
That would fit your loose desire for a Thailand - Laos - Thailand "sandwich" trip, but use your entries wisely and don't waste them for "a few days" on a whim.
When I checked a few years back it was possible to get tourist visas in neighbouring countries without too much hassle (Malaysia was one that was mentioned that was hassle-free, I remember) but I am not sure how a Thai border guard would treat back-to-back tourist visas in your passport these days. Maybe two within a few weeks/months of each other would be acceptable, but I really don't know - I'm just speculating. Something to be aware of, anyway.
Have a good trip and keep popping in the forum - lots of great info and helpful people here.