Good question! Sugar and caffeine are the cheap and reliable ways for me, but, like you, I don't want to rely on them.
Completing tasks, generally. So, not exercise specifically, but a Parkrun is a thing to tick off the list and work towards the next milestone. I wanted to read more this year, so I have an app where I record what I've read, and how far through each book I am. Like others, Duolingo.
... I have an app for housework tasks, and a Garmin to record exercise/work towards particular challenges. I use You need a budget for financial management, to track and manage spending.
Basically, I've gamified those tasks I want or need to prioritise 😂
More generally, time outdoors, particularly at this time of year if it's sunny, meeting up and having a laugh with friends, travelling to new places, really nailing a paper I'm giving. Those are all bigger highs but can't necessarily be relied on to give me a lift at 3pm when I'm in an a slump.