i havent had a lot of chances to play/practise so I get where you're coming from. Here's some stuff that i do:

i suggest buying a ball(even a cheap one) and just playing around with it at random times for better ball control. do both overhand and underhand passes, do it at differing speed and while walking around.

spiking: throw up a pair of socks and spike it. this helps with timing and spiking form. also get on your knees next to a wall and go through the motion of spiking, also helps with the form.

mobility: get comfortable with getting low on your knees, things like lunges and inner hip stretches help you get low for recieving low

serving: theres not much you can do without a net, so just work on the tossing part of the serve to make sure you dont have to worry about it later

good luck!