Looking to upgrade the audio on my wife's 91 Convertible without going overboard..

I bought a Kenwood DMX4710s din.
I was thinking of just replacing the dash speakers with 4in round speakers and upgrading the rears to a 6.5's with a tweeter. Figured I could use those adaptors that Screaming Chicken sells.

Is that about all I can do that's simple? I don't really want to get into subs, amps...

I was debating using q logic kick panels for the front and going with 5.25's... But not sure about the foot space. Would like to drive a car with it first and see and that's not likely...

If I'm on the right track, what are some good speaker suggestions?

 

I put Infinity Reference 4x6 in the dash and 6x9 in the sails and have been happy with them.

 

Unless you plan to use an amplifier I would recommend the 4" dash speakers and 6x9 speakers in the back. The separates work better with a subwoofer and amplifier, but without those two things 6x9 would be better rear speakers. With a convertible a good amplifier would be helpful for turning up the volume with the top down. The Rockford Fosgate speakers work well with radio power. Yes, a 5.25 would give you slightly better mid range compared to a 4" but I don't believe it would be worth the added expense and giving up potential foot space. The 4" speakers currently available work great for front speakers and it would be difficult to tell the difference between 4" dash speakers and 5.25 kick panel especially without a large amplifier. We have dash speakers and rear speakers available to the links below:

Dash Speakers:
1982-92 Camaro/Firebird Dash Speakers – Screaming Chicken
Rear Speakers:
1982-92 Camaro/Firebird Rear Speakers – Screaming Chicken

 

I wonder if you would benefit from the loudness of Polk speakers. I put some in a TJ for my dad. On a headunit with power ratings like yours. He likes them. He mostly listens to rock music. Polk's have pretty high sensitivity (more efficiency). Probably why a lot of the reviews are from motorcyclists.

You should stick with coax speakers over components because you have no amp.

Not sure which 6x9" can fit convertibles. A big 6.5" (as in measures slightly wider) or 6.75" should be decent enough for bass.

Since people hear music differently, the best way to choose speakers is to listen in person. Bring music liked and well known to compare speakers.
If unable to visit a speaker wall, crutchfield *don't buy from them* has a music preview feature. It works OK if you have decent headphones. It will ask what headphones you have in a drop down menu, and let you choose your own music or use their samples. It at least gives you an idea of which ranges get emphasized by each speaker brand. You can kinda get a sense of what genre they're for, female vs male vocals, which drum kit pieces emphasized, etc. Leave the option to normalize volume off and you can hear volume changes (sensitivity) between speakers as well.

And then order the speakers your wife likes best from screaming chicken.

 

Your correct, I forgot all about that when OP asked about the adapters. Unless a previous owner has modified the openings a 6.75" would fit a lot easier. You don't have much depth, but as long as the speaker is less than 2" on the depth it should fit. Kenwood makes a good 6.75" that we sell for convertibles that we sell at the link below:
1988-92 Convertible Camaro/Firebird Rear Speakers – Screaming Chicken

I also agree with above post on the Polk speakers, I just installed a set recently and they have been very impressive. I installed them on a 1967 Firebird, but uses the same size dash speakers (4"). I have a good size amp on them and I was replaced a set of 6.5" speakers in the kick panels, much better. We have their dash speakers on our website. You could use their 6.5" speakers in the rear but would possibly take some modification since your opening is 6.75".