Consider rewarding us for time spent playing the game. Your sponsors want contact hours right? Consider Extra spins of the wheel for every 100 minutes of play on the game. Add in special prizes that can change from day to day. An example for Fishdom might be a special prize could even be a small crab, clam, urchin, coral, starfish or lobster, perhaps an octopus, squid or even jellyfish; extra jackpots...
For Gardenscapes it could be some birds that occasionally fly into a tree or eat something off of the ground. Two of the same birds could nest and then none could win a baby canary, mockingbird or whatever. There could be a squirrel or whatever.... Each of your games could allow for this.. As one plays longer increase the likelihood of winning better prizes by eliminating those of lesser value.