What are some things that can be done to make the most out of very limited gym time?
Agree wholeheartedly with Get It Wright on need for plan. But if you are new, what's that plan look like?
Start with a plane that is set up carefully to a known, correct static condition.
For a fairly conventional plane (and if you are starting you should be flying a fairly conventional design) most are close to flying correctly with the following set up.
Balance point should be at rear wing post for say a 10 cm chord wing to last years rules. 1-2 cm if front of rear post for a 12-15 cm wing. 2-4 cm behind wing for a 5 cm bonus wing.
Wing level with motor stick (I use top of motor stick for my consistent reference on angle of attack) fore and aft. Offset about 1-2 cm to left. Right wing level fore and aft with motor stick, left wing washed in (leading edge high) about 1/8th inch.
Prop should be pointing SLIGHTLY (2 degrees or so) left and down.
Tail should be flat with respect to itself. Tilted slightly with respect to wing so left tip is higher than right. Tail boom should be offset to left about 1-2 cm at very end from line of motor stick. Leading edge of tail should be 2-4 mm lower than trailing edge with respect to wing level (or again, top of motor stick).
CHECK THESE THINGS BEFORE EVERY FLIGHT! The exact settings may change based on your data, but there's a reason pilots use checklists!
Start your first flights with just enough winds to turn the prop smartly, say a couple of hundred turns. You are essentially going to glide test the plane. Launch at shoulder height and observe plane. You are looking for a steady, slowly descending flight with a left turn. Here's where that list of trim scenarios mentioned is useful. If the plane stalls, what do you do, etc.
Once you get that nice steady turn and slow descent, start adding turns. Watch for weird changes in behavior, typically means you've broken something, you went to extreme on some setting at an earlier step, or your plane is too floppy. Correct small errors in behavior with small adjustments before moving on. Change one thing a a time! Again, a list of trim scenarios and how to correct is useful here. If no big problems, by the time you get to 1200 turns or so (depending on rubber width and prop pitch) should should be climbing nicely to the ceiling, circling a few times, and descending slowly.
With that you should be able to get to 1.5 to 2 minute flights fairly quickly.