Personally I wouldn't spend 5 days at the beach in such a short trip.

I think heading north to CM and CR is a good idea though. I visited an interesting cave shrine near CM last year on my visit. Lots of nature stuff around there as well plus tons of old wats. And there are elephant camps up there too. I really liked Siam Insect Zoo near Chiang Mai. Lots of butterflies begging to be photographed. On my first trip to Thailand I really enjoyed Doi Tung and Mae Fah Luang Gardens which is in Chiang Rai province. In Chiang Mai there's also the Rajapruek Royal Park which I quite liked too. I happened to be there on the King's birthday last year and there was all sorts of stuff going on for those celebrations.

It somewhat depends on when you are going. As jiejie always points out, you don't want to be in that part of the country when the slash and burn burning is going on.

For old temples you have a couple of options. Ayutthaya is a relatively short drive from Bangkok and can be done as a day trip (as long as it's not flood season). Or Sukhothai is a bit quieter and frankly more impressive with the temples around the town plus nearby Si Satchanalai park. If you happen to be there at Loy Krathong it makes the experience more interesting IMHO, albeit a lot more crowded.

Another possibility would be going to Isaan. There are some old Khmer temples there like Phnom Rung. Or Phimai which I really liked a lot.

I don't know how many SPG point you have that you are looking to spend. Some of the more out of the way places will not have SPG properties, but there will be cheap alternatives pretty much everywhere. The great thing about Thailand is that hotels are relatively cheap even at the higher end of the market.