Anyone who has played music in a live environment will have had to do some rehearsal beforehand. As someone who has recently returned to the live arena some years after last playing with others, here are a few things I remembered and that will help you if you’re getting started, or getting back into that world.
Your guitar pedal or keyboard worked fine in 2012… but does it still? Has it sat in a damp garage for a couple of years? Electrics and electronics are susceptible to environmental conditions as well as knocks, and the last thing you want to do is hump your gear all the way to the rehearsal room to find that - oops - the fuse has blown, or the thing has horrible crackling on its stereo outputs. Ideally you’d check this several days before not on the day of the rehearsal, as these kinds of problems can take time to fix.
Sitting down with some other people and saying “uuh, what now?” isn’t conducive to good vibes or effective performance. It’s not necessary to have a military map of exactly what you need to do (unless you want one), but it is advisable for someone to have emailed across a few tunes or ideas or even just a bit of direction before everyone turns up. That way, you’re not all completely flying by the seat of your pants.
You might be rusty, having not played for a long time. And practising an instrument by yourself is always helpful, but it’s not the same as playing along with other people - who may also themselves be reacquainting themselves with the whole process. It’s fine when you start to just kick some ideas around and get a feel for each others’ playing and style. After a few rehearsals, sure, you should be starting to firm things up a little more, but at first it’s fine to just take it fairly easy.
Rehearsal spaces get a real hammering with all the people that pass through them from death metal bands to drummers who need somewhere to make noise. As a result, owners tend not to put really top end PA equipment in them, since it can get thrashed. So keyboard players in particular (I’m looking at you) might want to take their own amp if they have one, rather than risking your keyboards distorting horribly when you try to make them loud enough through the house PA. You might get lucky, but you might not.
Most smaller gear like pedals and recording devices can be powered by batteries or an external PSU, but batteries run out. It’s always worth having a switchable PSU with you - with switchable voltage and swappable connectors, in case you ever run into difficulty. These are inexpensive and can ve adapted to power and fit most types of music gear like pedals and other smaller units.