Here are some off the top of my head:

In "Tell it Like it is" Peter says he got into another fight with Buddy Hinton ("A Fistful of Reasons")

In "Greg Get's Grounded" Greg reminds Rachel of the time he was forced to take Bobby to the drive-in movie ("The Big Bet")

In "The Hustler", you can see boxes that were also used in another episode. I think those boxes are either "Safe" laundry detergent or from Driscols Toy Shop ("The Hero.) (Sure someone will clarify.)

Boxes of Safe are not only used in "And Now A Word From Our Sponsor" they also appear in "Law and Disorder."

Greg is seen reading the same baseball book in "Juliet is the Son" and at least one other episode. I think Carol also gets the same book when she tries to teach the boys baseball when she and Mike switch household responsibilities. ("The Grass is Always Greener")

The same schoolbooks are used by the Brady kids often throughout the series.

The classic "Her Sister's Shadow" scene with Jan refusing to accept her Honor Society Award because of the judging error on her essay contest with the audience clapping is old stockpile footage that was used when Greg and Marcia both run for Student Council President ("Vote For Brady")

Greg's girlfriend Rachel appears much earlier in the series as one of Marcia's friends in "The Slumber Caper."

Marcia's cheerleading rival Kathy Lawrence, seen in the classic "My Sister Benedict Arnold" episode is the SAME actress who appears in the very next episode, where she is just recognized as "Kathy" ("The Personality Kid") and NOTHING is made of it!

This always bugged me! They could have either found a different actress for the role or not shot those two episodes so close together in sequence! In this above example, this is a case of obvious character duplication, NOT talked about in other episodes!

Jack

There were other actors who played different characters in different episodes.

Marcia Wallace played a teacher in the Davy Jones episode and then played a saleswoman in the department store where Jan bought her wig.

And the guy who played the principal of Jan's school in "My Sister's Shadow" ended up in 3 or 4 episodes, playing different characters (his name was C. Lindsay Wrokman, and he was in My Fair Opponent (1972), playing Mr. Watkins; Her Sister's Shadow (1971) playing the school Principal; Lights Out (1971) , playing a schoolteacher and "To Move or Not to Move" (1970).. I guess he played a prospective house buyer

And, of course, there's the legendary Jim Backus, who played Zaccariah T. Brown, the prospector who thought the Brady's were trying to jump his claim in the one of the Grand Canyon episodes... and then he shows up as Mike's boss, and he gives the Brady's a pool table as a gift.

Fairview quarterback Jerry Rogers (played by Chris Beaumont) also played (1) an older boy who grabs the attention of the girl that Greg was trying to impress in "Our Son, the Man"... then he played "Eddie" who sold Greg his first car, a lemon... then he played Greg's older friend (who was out of high school and on his own living in an apartment in college) in "A Room at the Top"... and finally, he played Jerry Rogers, Fairview quarter back, in "Quarterback Sneak"

And the guy who played the principal of Jan's school in "My Sister's Shadow" ended up in 3 or 4 episodes, playing different characters (his name was C. Lindsay Wrokman, and he was in My Fair Opponent (1972), playing Mr. Watkins; Her Sister's Shadow (1971) playing the school Principal; Lights Out (1971) , playing a schoolteacher and "To Move or Not to Move" (1970).. I guess he played a prospective house buyer

Mr. Workman in "To Move or Not To Move?" was actually the Realtor.

Jack