lol yeah, it always seems obvious after someone points it out.
But the type of drills you should do largely depend on your weaknesses. To try to cover everything in one fell swoop, I'll list an example drill for several shots.
Volley
This is a fun one, at least in my opinion. As with most drills, this requires a hitting partner. Stand at the net, as close to it as you feel comfortable (at least halfway between the service line and the net). Have your partner stand at the baseline of the other side of the court. For beginners or players with weak volleying skills, have your partner hit directly to you, to allow you to focus on just volleying. After you get more accomplished at the net, your partner should start trying passing shots. This carries the double benefit of working on your net game as well as working on your partner's passing shots.
You can set up a sort of mini-game format with this drill by keeping score. The format I use is first player to 7 points wins, no lobs allowed.
Groundstrokes (Forehand & Backhand)
The backbone of any tennis player's game is his or her ground game. As you might have guessed, the only way to improve groundstrokes is to hit them, over and over. For this one, I would recommend that you and your partner stand on the baselines. At first, just hit back and forth, trying to keep the ball in play. Once you get to the point where you can do this consistently, start focusing on control. Make certain areas of the court off-limits. (For example, both players are required to hit it cross-court.) Another way is to only allow forehands or backhands. (This one helps if one side is considerably weaker than the other.) There are a zillion different variations for this drill.
Again, this drill lends itself to a mini-game format. Use your imagination to think of fun ways to incorporate both forehands and backhands.
Serve
The serve can be a huge weapon or a glaring weakness, depending on how much you develop it. In my game, my serving either makes me or breaks me. If I have a good service day, I usually win. If my serve is off, I'll usually struggle through the match.
This is the one drill that is best done without a partner (though make sure you have plenty of balls!). If you're a terrible server, just try to focus on getting it in the service box. (If you're
really bad, try standing at the service line instead of the baseline. Once you get consistent from there, gradually work backwards until you're at the basline.)
After you're able to consistently get your serves in, try working on accuracy. Set up targets (such as plastic cones or small pyramids of balls) and try to hit them. Also, make sure you spend time developing a reliable second serve. (Using more spin on the second serve is usually prefered over a weaker flat one.)
If you consistently work on all three of these drills, you'll be mopping people around the court in no time!
Let me know if you have any questions or need any clarifications.