Depends what you want to get from the shoot. Split the time you have so that you're not focusing fully on one thing.
If you're not comfortable with flash or you don't know quite what you'd do with it, don't mess around all the time with it
Just get a nice range of shots and try different things out in moderation - dedicate a bit of time to natural light/indoor/out/flash/mixed/etc. Good thing is if it's just practice/with a friend, then have fun, no pressure!
A good idea though, is to plan out how you're going to go from one shot to the next, and also what you/your friend want to get out of it - because theres a load of things you can do and it's easy to get side tracked. Whether it's planning in detail, or just roughly in your head, it's always good to plan. For example you could start off indoors, find a nice big window while the sun is still out, position your subject accordingly and you can get some nice natural lit headshots. Then get some natural full length shots, different view points, angles etc. Then add in a bit of flash to pratice, try bouncing for fill flash, whack up the shutter speed and see what you can get with just flash, then as the day goes into the golden out work your way outside and try some cool location shots.
High/Low-key with Flash
One thing you could try is finding a plain white-ish wall, setting your flash behind the subject or off-camera somewhere, and blowing the wall to white so you just get a super-clean shot. It's pretty doable with just one flash, but you might need to max it out depending on the ambient light you're getting and the sync-speed of your wireless triggers. Use a window or some other light source as your main subject light, or even the built-in flash with some tissue paper on etc, and it becomes a quick and easy way to get some nice shots that look like they were done in a studio - with a nice clean background.
The reverse of that is also pretty easy, where you make the background dark/completely black - for that you just up the shutter to cut out the ambient light, and position your flash so it's hitting the subject, but angled away from the wall, or just move further away from the wall.
Think about the type of light you want, and what would fit the subject. Hard/bare flash works great for males because it gives strong/defined shadows, but it's not flattering to females. To get a softer light, you need a bigger light-source, that's where a soft-box would come in. You can get a good effect from a white bed sheet or cloth draped over a couple of chairs or something too - just put the flash about 1 foot behind it at it's widest setting and you're half-way to a soft box.
Check out these for some examples:
Outdoors
Also bear in mind that if you decide to outdoors early in the afternoon, it's still gonna be pretty bright, so your flash is going to be fighting with the sun if you're trying to get that 'darker background' look in a non-shaded area - so golden hour is gonna be better for that sort of thing. You probably won't be able to get that much separation of subject/background brightness on a very sunny day, because the wireless triggers will max out at say 1/160 maybe, and the 1 speedlight won't be able to compete. But, try different things!
I think for outdoors, the location and style of shot is going to be more worthwhile practising than trying to use flash. You might want to avoid using direct sunlight as it can be quite harsh - pay attention to the subjects eyes, if they're getting irritated and blinking then turn them so they're out of the light a bit. Also look for white walls/signs to use as natural reflectors to act as fill light - the larger the source of light, the softer it'll be.
Try some silloutets outdoors too, get your subject back-lit by a fading sun, move around to get some nice flare, and boom, arty-sunset shot.
Check out:
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Oh, and if you wanna go crazy - grab a tripod, a torch, somewhere dark, and do some light-painting like this!
They make it look so easy ...
Hope that helps a bit,
Good luck, tell us how it goes!