Dowels and biscuits are length-preserving, cheap and common ways of generating some long-grain glueing surface. They're not quite as strong as other solutions, but usually entirely sufficient.
For dowels, you only need a drill and transfer plugs (or a dowel jig, which is more expensive). If you don't even have transfer plugs, you can improvise them with a dowel and a nail, or even with a dowel and a felt marker (no joke, done that before...). For biscuits, there is a special kind of tool, the "biscuit joiner", which makes sawing the slits in an automated process (not exactly the Mount Everest of traditional craftmanship, but it works very well).
Tongue and groove is very slightly more work than the above. If you have appropriate tools (a router will for example do, but manual tools are perfectly fine), it's a matter of minutes. I'm using a plow plane for that since taking the router out of the box and assembling/adjusting everything takes about as long as it will take me doing the tongue and groove with the plow plane, and I don't need hearing protection, nor is there a lot of fine dust. Hand saw and chisel certainly works, too, for the patient and dedicated traditional craftsman.
The tongue can also be loose (a separate piece of wood) which is basically like a single large biscuit going all the way through. The advantage of a loose tongue is that you do not lose length. Mind that the external appearance is different, you'll see end-grain at the tongue, which can be undesirable but can also be very desirable (indeed sometimes people deliberately use a completely different wood for contrast, it can look pretty cool).
Then of course there are about two dozen variations of joinery that you could use, all of which do lose some length, however.
What to use is hard to answer, as it depends on how much work you want to put into this, how stable it has to be, in which direction the expected force will act, and what "look" you want.
A simple lap joint, for example, is perfectly fine if you do not have any "special needs" on stability. It can be made easily and quickly with no tools but saw and chisel, or with a rabbet plane (or, of course, if you have a router, with the router).