I've been enjoying playing Rikudo Puzzles on android - https://play.google.com/store/apps/details?id=com.rikudo.numbers
It's like Simon Tatham's Signpost game, but on a hexagonal grid. You have to place numbers in the cells to form a contiguous path which fills the entire board. I don't get much out of the timer and stars for finishing quickly - I just like solving the puzzle. Placing numbers can be a bit fiddly - it's very easy to accidentally switch to counting backwards, or miss out a number.
@christianp That's more or less how I found that app, too.
Seems to be how mobile games are at the moment. I've been playing Gerrymander https://play.google.com/store/apps/details?id=com.studioBlimp.Gerrymanader&hl=en (draw constituencies on a map such that you win) and it's very clever but I wish there were a 'pen and paper' style version where there's one solution per puzzle and it takes more than 30s to find it.
I get how that might have more limited appeal but equally pen and paper style puzzle games do tend to run OK on more devices, so...