The Lions finished up their three-day mandatory minicamp on Thursday. The next time we're expected to see a full 90-man roster on the field together is the start of training camp later this summer.

It was a good three days of work in Allen Park with a lot of competitive periods and situational work.

Here are five takeaways I had from the last three days of practice:

1. Work and play

There's no rule that says teams aren't allowed to have a little fun playing this game. Former Lions coach Wayne Fontes used to say that you play football, you don't work it.

We've certainly witnessed that the last three days of minicamp under head coach Dan Campbell. Campbell is a former player and competitive guy, and when it's time to work and get things done, he's all business. But he and his coaching staff, including a lot of other former players, have been able to infuse a lot of competition and some fun into practice as well. Whether it's medicine ball tennis with the big guys, races among the skill players, some defensive line tag drills or even letting coaches join in a race -- there's a different feel around Allen Park under Campbell.

Football is a game, and even though the NFL is a business, teams can have a little fun and work hard at the same time.

2. Swift could be a real threat in the passing game

Running back D'Andre Swift got a lot of reactions from teammates and coaches this week for some of his catches and route running ability.

We saw the kind of impact Swift can have in the passing game a little bit last year after he caught 46 passes for 357 yards and a couple scores. But his speed, shiftiness and ability to cut on a dime have really stood out in the passing game this week, and really all offseason. He's put some defenders on skates these past few days and has caught just about everything thrown his way.

Swift and quarterback Jared Goff seem to be building a pretty good rapport. Offensive coordinator Anthony Lynn is lining Swift up all over the field, and using him in a lot of different ways in the passing game. Lynn said his offense is all about getting his skill players the ball, whether it's handing it to them or throwing it their way.

Swift said this offseason he can be a 75-plus-catch player if given the opportunity. After watching minicamp the last three days, I believe him.

3. Receiver depth chart is wide open

General manager Brad Holmes completely retooled the wide receiver position this offseason, and Lynn said there's really no established No. 1 guy there right now.

Veteran free-agent acquisitions Tyrell Williams and Breshad Perriman are expected to play significant roles, but both the depth chart and roster spots seem to be wide open at this point.

We saw Victor Bolden, a practice squad player the last couple years in Detroit, have a really solid minicamp. Second-year receiver Quintez Cephus was also good the last three days. Rookie Amon-Ra St. Brown showed off some position versatility and good route running. Where could those three fit into the mix?

It will be fun to see how the pecking order plays itself out at the receiver position come training camp.

4. Competition in secondary

Amani Oruwariye and Quinton Dunbar have rotated first-team reps opposite Jeff Okudah this offseason. That should be a pretty good competition for one of the starting spots on the outside at cornerback. Oruwariye and Dunbar showed some good things in minicamp, especially on Tuesday, when I thought the defense was the better unit in competitive periods against the offense.

Can third-round pick Ifeatu Melifonwu work himself into a package role? Mike Ford did some good things in the nickel this week. He and Corn Elder are expected to compete for the slot corner role.

At safety, Will Harris and Dean Marlowe worked into first-team reps next to Tracy Walker, who said he really likes this new split-safety scheme defensive coordinator Aaron Glenn has brought over from New Orleans. Campbell said this week he likes the talent and depth at safety and is looking forward to seeing which two players emerge as starters there.

Detroit ranked last in the NFL last year with opposing passers having a 112.4 passer rating against. They have to be better in the back end of their defense, and that means we'll need to see some improvements from a number of different players.

5. Waiting on the big men

OTAs and minicamp are conducted in helmets and shorts, which means there's little we can learn from the run game and the offensive and defensive line play. There wasn't any hitting or one-on-one pass-rush drills, so the evaluation of the big guys won't come until training camp.

Detroit's offensive line is expected to be one of its strengths with the addition of No. 7 overall pick Penei Sewell at right tackle. It will be fun to watch him in action when the pads come on, but we have to wait until camp for that.

Detroit put significant resources into improving the defensive line, particularly in the middle, with the additions of veteran Michael Brockers and rookies Levi Onwuzurike and Alim McNeill. Just how improved will they be upfront on defense? All eyes will be on the big guys to start training camp.