Try this
- Acknowledge the invitation and decline without burying the answer in compliments.
- Offer an alternative only if you want one and can name a realistic option. Otherwise let the declined invitation be complete.
A worked example
“Thank you for including me. I won’t join the hike, but I hope you enjoy it. I’d like to hear about it afterward if you feel like sharing.”
An invented example to adapt, not a real exchange or a script you have to follow.
Before you leave it here
Do not suggest another date just to make the refusal sound kinder if you do not intend to follow through.