Here is some gear I use myself, and use with my clients.
Jill Miller's Yoga Tune Up Alpha Ball
Highly recommended. Hands down the best piece of massage and self-care equipment I own. I've bought them for clients just to ensure they do better self-care between sessions.
SlingShot Hip Circle
Great for "monster walks," sideways walks, and hip abductor (aka, spreading your knees against resistance.) Useful for queuing "knees out" position on squats and deadlifts. I use a size L hip circle with most of my clients, even those below the weight listed for size L. These wear out, but they're better than the jury-rigged band or theraband solutions I used before.
WODBand Resistance Loop
As the Slingshot Hip Circle above. Equally good, often lower priced. I have both and use them both.
If your gym has standard 1" dumbbells and barbells, a pair of these will go very far. You can use them to make thick-handled implements for farmer's walks, friendlier pressing, rows, etc. They don't do well if you've got dumbbells that get progressively thicker as the weights go up. They also make great handles for bands and looped tubes!
41" bands, especially the CFF #0 and #1, are a stable in my training. I use the #0 and #1 for standing Ws, band pull-aparts, rotator cuff strengthening, and more. Coupled with a Fat Gripz handle, they make great Pallof Press tools. Get a #0 and a #1 and throw them in your gym bag and you won't regret it.
WODFitters, EliteFTS, and Iron Woody make similar bands. Match the diameter, not the colors! I've had clients buy "one of those orange ones" and get a thick, heavy band meant for pullup assistance when they wanted a thin band for rotator cuff rehab.
My ideal tool for self-massage on the front of your hips, and a useful tool for performing hip adduction. Sometimes used for actual soccer.
Beaded Jump Rope
Before you spend $500 on a treadmill or an elliptical or $150 on running shoes, spend $10 on a jump rope and use that 3 times a week. Excellent portable cardio equipment. Boxers swear by and swear at jump ropes, and you'll do the same if you put the effort in. Don't get discouraged, cut the length down to the proper one for you - stand on the rope, the handles should reach your armpits - and put the time in. I like that Valeo one, mine has held up for years, but any rope you'll jump will do. Even a bit of rope will do in a pinch.
Great for body weight rows, suspended pushups, mountain climbers, and even as ersatz dragging sled handles. The TRX ones are good but pricey, so also consider the Jungle Gym, EliteFTS's metal Blast Straps, or a good set of rings. I use all of those.
Pretty much any brand will do. 36" is much better than either an 18" or 12" foam roller, but if you need portable, 12" is the one to go with. Use these for self-massage and as a prop for yoga positions, stretches, and mobility drills.
Muscle Floss Bands
Reviewed here: WOD Nation Muscle Floss
It takes a little practice and watching some videos to get the benefits of joint/muscle wrapping, but it's worth it. These bands completely changed how I deal with joint and muscle pain or restrictions with myself. I've used them less with clients, but I have used them, and it's the same result - a substantial and immediate positive change in the way a joint is operating.