There are a lot of great things that you can do on behalf of your brothers and sisters in Christ. You can call them to check on them, write them a card to encourage them, hug them whenever you see them, invite them into your home for fellowship, perform some random act of kindness, comfort them in times of grief or trouble, etc., etc. We are commanded to do all of these sorts of things (cf. 1 Cor. 12:25; Gal. 5:13; 1 Thess. 5:11; Heb. 10:24; 1 Pet. 4:8-10; 1 John 4:7-16, etc.).
Above all of these (and more), there is something you can do for your brethren that will help them more than you can even know or measure. Paul summarized it in this short verse, “Brethren, pray for us” (1 Thess. 5:25). There is nothing better, nothing more powerful, nothing more enduring than taking the name of your brother and sister in Christ to the throne of the Almighty God. (And it is so easy: anyone can do it; you can do it on behalf of anyone.)
Then, if you want to make their day, tell them you are praying for them (cf. Phil. 1:3-4).