You can always either do pullups on a tree branch or buy a doorway pull-up bar.
Also, TRX style bands are great for several exercises.
I love TRX rows, triceps extensions, curls, and rear delts.
TRX bands can be expensive, but I made one real cheap out of steel chains and quick links from Home Depot (with PVC pipes for handles).
Anyway, without the TRX bands:
Back you can do pull-ups (assuming a place to do them), chin ups, various barbell rows (overhand and/or underhand), and dumbbell pullovers on the box, assuming it’s a suitable height. Maybe one-arm dumbbell rows too, supporting on the box, if it’s a suitable height. Also maybe rows with plates on one end of the bar and the other propped in a corner if you have a corner you don’t mind potentially scuffing up a bit.
Biceps you always have barbell curls. No idea if the only plates you have are 25s and 45s. If so, then your only options for barbell curls are 45, 95, and 135. You can always also hold a 25 lb Plate in each hand and do curls with them, or hold 1 45lb Plate and do two-arm curls with them.
Same for triceps extensions; either bar with 45 or 95, or just holding the 25 or 45 Plate.
You may also be able to set up the plyo box to do skull crushers with, if you have your friend hand you the bar before and take it after the set. Same with light close-grip or reverse grip bench press if you trust him to spot you and set up and take it.
Shoulders you have shoulder press, push press, clean and jerks, shrugs, and front raises with a 25 or 45 lb Plate. Should be more than sufficient.
Chest you have all sorts of push-up variations. Normal, close grip, diamond, one-arm, feet up, clapping, one hand up on a medicine ball or dumbbell, etc.
Legs you can do front squats if you’re comfortable cleaning a weight up into position and squatting like that. Same with lunges, with a light weight. Or you can just hold plates and do squats or lunges. Depending on the height of your box, you can do split squats too. Then you have deadlifts and RDLs. And you can do those things where you get on your knees and your partner holds your feet down while you lower and raise your upper body up and down with your hamstrings. You can always do controlled negatives and push yourself back up if they’re too hard.
Then for forearms you can do barbell wrist and reverse wrist curls.
As for putting these exercises into a logical and coherent workout plan, that’s a little more work. Maybe see which ones work well and feel good/safe first for a workout or two before going all out and making an actual routine?
