What's a cheap starter kid to get started with activations, such as #POTA, #GMA, #SOTA and the likes?
I think the answer depends on whether you're able or willing (and happy) to dive into #MorseCode.
If so, there are cheap, but rather performing QRP monoband CW rigs out there, light enough for backpack use.
For one particular example with a good reputation, see the QCX of Hans Summers / QRP Labs, most recent version at https://shop.qrp-labs.com/kits/qcxp .