Tell them...one-on-on...in small groups...the entire company...over, and over again that you genuinely, authentically appreciate what everyone is doing - individually and collectively. The biggest reason for people leaving their current company - even in these turbulent times - is feeling that they were not appreciated at work - and I don't think any 'gift' or other 'thing' recognition can replace having a real conversation that expresses your appreciation.
Next is time...particularly in start-ups, where things are hectic and totally focused on launch dates, short product cycle times, time to market, etc...people value having time away from work. Be sure you aren't scheduling any major milestones or due dates and give people the gift of time. Make sure you are connecting your appreciation for what they are doing with the gift.
Last, if you want to do something more tangible...it's a matter of budget - how much do you want to spend??? I'd be cautious of the traditional 'Holiday Party' - it's an opportunity for workplace mischief to get out of hand if alcohol is involved. Two books to check for ideas: 1,001 Ways to Reward Employees and 1,001 Ways to Energize Employees - while I don't like the word 'employees' in the title...substitute People and these books are full of ideas that will fit any budget.
Happy Holidays...and Thanks for all you are doing to get a start-up off the ground...Ben