Just curious, is it okay with Adsensse T&C if we place a donation button on a site carrying adsense?

Yes it's fine to put in a donation button. The only affiliate I know that would work across any niche would be Amazon, but I would manage my expectations as regards earnings. Affiliate marketing is different from AdSense - you actually have to *sell* it, which of course, you can't do with AdSense.

Thanks for the reply. I seriously considered putting in a donation button on my main site until couple of days ago, but the last couple of days have been really good. The day before yesterday was one of the best in a few years in terms of CTR and CPC and yesterday was also better than usual. This is the kind of CTR and CPC I used to get. If it continues like this then the donation button will obviously not be required. The donation button will always be a last resort. I will try out Media dot net before the donation. But my experiences with them has been like many others. They start out well before the CPM gradually declines to a point where you get very frustrated, made worse by the fact that reporting is done every 24 hours (some days even two days almost). But their ads look kind of nice and the link words in the ad line up really well to the content on the page.

Maybe Amazon ads as backup?

I've used Amazon long before AdSense or even Google itself were invented.

Amazon still continues to be a good earner for me.

But as a backup? I would only use it in a way where the backup redirects to a specific page on your site where you highlight a specific Amazon product.

If you ever considered using a generic Amazon banner ad? Don't - they are notoriously lousy performers. Might work for Mr. Amazon, but never for an Amazon Associate.

Ideally Amazon - as with AdSense - works on a page where you discuss the ins and outs of "white widgets with green stripes". Hopefully AdSense would serve you up an ad relevant to "white widgets with green stripes". But for the Amazon backup?

Personally on that page I would have already placed a direct link to Amazon to the specific product "white widget with green stripes" in their inventory.

An alternative AdSense backup ad for it to Amazon? Can be done with difficulty but really you would only be referring yourself ideally back to that page.

Funny thing about Amazon. You might sell one "white widget with green stripes" for say $1.50, then to the same buyer sell $600.00 worth of merchandise wholly unrelated to your site. In a recent example I sold 18 sets of very expensive door locks. Don't ask because I don't know - 18 doors?

I seriously considered putting in a donation button on my main site until couple of days ago

That largely only works where you have a dedicated, and repeat following, often backed up with your own discussion forum.

Actually? I haven't seen it work well over the last 15 years - before that yes - but the internet was much smaller back then with many special interest [DIY?] sites.

Even prominent, real quality news outlets struggle with garnering donations today.

Donation or patreon-like button by itself without any supporting material is a horrible implementation. You will have to bring tons of attention (Like wikipedia bombarded for donations a couple of times per year), and write up a bunch of related information on how "donate" can help website users. Meaning, repeatedly asking for it, beg it, all over your site. Most of sites that I see some success with donations all have to end up doing this tactic. Not worth it, for most people anyways.

The only ad conflicts that I see is a alarm with adsense is non family friendly ad networks that focus on #*$!, or popup/unders. Everything else is generally OK. I use criteo and sovrn, or other CPM networks that allow you to set floor price. I usually try new networks out when I get email invites or hear them on the forum here. But I generally place them as first tier, and fill adsense after them, since adsense has a much better coverage (at lower RPM). I generally find that such setup does not hurt adsense RPM...much. Using a tiered approach increase overall revenue by 5~20% (at some speed / loading time hit due to ad passing back).

Unfilled Adsense backup can finally be used with something like affiliate banners aka amazon, etc. I generally expect very little payout at Tier 3/4 in the waterfall. It is usually at 1~2% of overall impression anyways, very low value. But they're more or less beer money, still look better than blank screens.